What is the ideal pH for a tropical fish tank?

It depends on your fish — but the honest answer for most community tanks is 'whatever your tap water is, as long as it sits between 6.5 and 8.0.' Chasing pH by adding buffers causes swings that stress fish more than off-target stable values. Neutral 7.0-7.4 suits the widest range of species. Soft-water specialists want 5.5-6.8. Hard-water fish like livebearers and rift cichlids want 7.5-8.4. Stability is everything.

What are the perfect water parameters?

There is no single perfect set — parameters must match the species you keep. For a general tropical community: temperature 22-26 C, pH 6.8-7.6, GH 6-15, KH 3-8, ammonia 0, nitrite 0, nitrate under 20 ppm, TDS 150-400 ppm. Match your fish to your local tap water rather than fighting chemistry.

How do I lower pH in my aquarium?

Gradually and biologically, not with chemicals. Add Indian almond leaves, alder cones, driftwood, and peat-based substrate — all release tannins and mild acids that naturally drop pH over weeks. Mixing RO water with tap water lowers both pH and hardness together. Bottled pH-down liquids cause rapid swings that stress fish more than the original pH did. If your tap water is pH 8.0 and you need 6.5, use RO mixing, not a chemical.

How do I raise pH in my aquarium?

Add crushed coral, aragonite, or limestone to the filter or substrate. These dissolve slowly, adding calcium carbonate that raises both GH, KH, and pH in a stable, buffered way. This is how rift lake cichlid and livebearer keepers in soft-water regions boost their water. Again, avoid bottled pH-up products — they cause swings without buffering.

What is GH and KH in aquarium water?

GH (general hardness) measures dissolved calcium and magnesium — it is what makes water 'hard'. KH (carbonate hardness, sometimes called alkalinity) measures carbonate and bicarbonate, which buffer pH against changes. High KH locks pH stable. Low KH allows pH to swing. Most tropical community fish want GH 6-15 and KH 3-8. Soft-water species want GH and KH both under 6. Rift cichlids want both over 10.

What is a safe nitrate level?

Under 20 ppm is ideal, under 40 ppm is acceptable, above 40 ppm sustained causes long-term health decline and suppressed breeding. Live plants and regular water changes are the practical way to keep nitrate down. If your tap water already contains nitrate (common in some UK regions — Anglian water is often 20-30 ppm from the tap), you will need larger water changes or RO mixing to hit the low end.

How often should I test my water?

During cycling, daily. For the first month of a stocked tank, weekly. Once the tank is stable (6+ months of zero ammonia and zero nitrite), monthly is fine, or when you see any behavioural change in the fish. Always test before water changes, not after — you want to catch problems, not confirm the fix worked.

Is UK tap water safe for fish?

Yes, once dechlorinated — UK drinking water is strictly regulated and safe for the vast majority of community fish. Treat it with Seachem Prime or equivalent to neutralise chlorine/chloramine. Check your regional hardness on your water company's website. The only scenarios where tap water alone is unsuitable are for soft-water specialists (discus, wild tetras, crystal shrimp) in hard-water regions (London, SE England).

What is TDS in aquarium water?

Total Dissolved Solids — a measure of all dissolved minerals, salts, and organic compounds in water, measured in parts per million (ppm). A cheap £15 TDS meter gives an instant read. TDS correlates roughly with GH and KH. Typical UK tap water runs 200-500 ppm. RO water reads 0-5 ppm. Most fish do not care about absolute TDS within reason, but shrimp and breeding setups benefit from consistent values — swings more than 30-50 ppm in a day cause stress.

What is the nitrogen cycle?

The biological process that makes aquariums viable. Fish produce ammonia. Nitrosomonas bacteria convert ammonia to nitrite. Nitrospira bacteria convert nitrite to nitrate. Nitrate is removed by plants and water changes. A 'cycled' tank has the bacterial colony established — ammonia goes to zero within hours of being produced, nitrite never appears, and nitrate builds up gradually between water changes. Cycling a new tank takes 4-6 weeks with fishless cycling. See our [first tropical tank guide](/care-guides/first-tropical-tank) for the step-by-step.

Why is my aquarium pH dropping?

Usually collapsing KH. As the nitrogen cycle produces acids and CO2, KH is consumed over time. Once KH drops below 2 dKH, pH becomes unstable and often crashes suddenly to 6.0 or below. Fix: larger water changes with your tap water, or add a small amount of crushed coral. If your tap water is very soft to begin with, you may need a KH supplement.

What temperature should my tropical fish tank be?

Most tropical community fish: 22-26 C. Specific species narrow the window — [neon tetras](/care-guides/neon-tetra) prefer 22-24, [cardinal tetras](/care-guides/cardinal-tetra) and [discus](/care-guides/discus) want 26-28, [corydoras](/care-guides/corydoras) do well at 22-24. Stability matters more than exact degree — a tank that holds 24 C all year round is healthier than one that swings between 23 and 27. Use an adjustable heater with thermostat.

How do I remove chlorine from tap water?

A water conditioner (dechlorinator). Seachem Prime, API Tap Water Conditioner, Tetra AquaSafe — all work on standard chlorine. If your water company uses chloramine (Thames, Anglian increasingly do), make sure the conditioner explicitly handles chloramine — not all of them do. Prime is my default because it covers both plus detoxifies trace ammonia. One dose per water change, follow the bottle's ratio.

Why stable is better than perfect

Fish evolved to handle gradual parameter shifts — seasonal, tidal, long-term. They cannot tolerate abrupt changes. A tank held at pH 7.6 constantly is healthier for a fish that 'prefers' pH 6.5 than one that swings between 6.2 and 6.8 every water change because you are dosing buffers. Osmotic stress from parameter swings causes more deaths than off-target-but-stable values ever do. Match your fish to your water, then leave the chemistry alone.

Aquarium Water Parameters & Chemistry Guide

Q: Do I need to use RO water?

Only if you keep soft-water specialists in a hard-water region, or breed species that require it. RO water adds cost, takes space for a unit, and wastes 3-5 litres to make 1 litre of pure water. For community fish in most of the UK, mineralised tap water is better than pure RO. If you must use RO, mix it with tap at 50/50 or 70/30 depending on the target hardness, and always remineralise with a product like Salty Shrimp or Equilibrium.

Water chemistry is the least glamorous part of fishkeeping and by far the most important. I have seen gorgeous tanks with premium equipment and expensive fish collapse because the owner never bothered with a test kit. I have also seen scruffy-looking tanks in grotty flats thrive for a decade because the owner tested weekly and understood the numbers. Equipment helps. Water chemistry wins.

This guide draws on data from FishBase^[1], Seriously Fish^[2], Practical Fishkeeping^[3], the British Geological Survey's UK hardness maps^[4], peer-reviewed work on chloramine toxicity in fish^[6], and 15 years of testing UK tap water from London hard to Scottish soft.

My core argument across this whole guide: stable beats perfect, every time. I will repeat it often because it is the single most counterintuitive thing about water chemistry, and the single most important thing to understand.

At a glance

The three parameters that matter most: temperature, ammonia/nitrite, pH stability
Test kit: liquid (API Master), never strips
UK water: soft in Scotland/NW, hard in London/SE, variable everywhere else
Dechlorinator: every water change, every time, no exceptions
Rule of thumb: match your fish to your water, not your water to your fish

Mistake I've made

The most expensive mistake I see weekly: someone discovers their pH is 7.8 out of the tap. They read somewhere that tetras "need" 6.5. They buy pH Down, dose it to hit 6.5, and see their fish go nuts for two days. Then the KH buffer runs out, pH crashes to 5.8 overnight, and half the tank is dead by morning. Next water change, they dose more pH Down. Cycle repeats. The fix was never to chase pH — it was to keep tetras that tolerate 7.8, or to use RO water to lower GH/KH/pH together in a stable way.

Good water chemistry means clear water, healthy fish, and zero drama

The three measurements that actually matter

Every fishkeeping forum argument can be distilled to confusion about which parameters matter. Here is my priority list, built over years of seeing what kills fish and what does not.

1. pH — the acid/alkaline balance

pH is a logarithmic scale from 0 to 14. 7.0 is neutral. Below 7 is acidic, above 7 is alkaline. Each whole-number change represents a 10x shift in hydrogen ion concentration — which is why a drop from 7.5 to 6.5 is a much bigger event for fish than the small-looking number suggests.

Most tropical fish tolerate pH 6.0-8.0, with individual species preferences within that range^[2].

Soft-water species (discus, wild-caught cardinal tetras, Caridina shrimp): 5.5-6.8
Community tropicals (neon tetras, corydoras, harlequin rasboras): 6.5-7.5
Hard-water livebearers (guppies, platies, mollies): 7.2-8.0
Rift Lake cichlids: 7.8-8.6

2. GH (general hardness)

GH measures dissolved calcium and magnesium — the "hardness" of water. Measured in German degrees (dGH) or ppm (1 dGH ≈ 17.9 ppm CaCO3).

Very soft: 0-4 dGH (Amazonian species, crystal shrimp)
Soft: 4-8 dGH (most tetras, rasboras)
Medium: 8-15 dGH (community tanks, most livebearers)
Hard: 15-25 dGH (mollies, guppies, rift lake cichlids)

Fish cope with wider GH ranges than they are often given credit for, provided the change is gradual. Keeping soft-water tetras in 12 dGH is fine; keeping them in water that swings between 4 and 12 weekly is not.

3. KH (carbonate hardness)

KH is the buffering capacity — how resistant your water is to pH change. Higher KH = more stable pH. Measured in dKH.

Very low KH (0-2 dKH): pH unstable, can crash. Soft-water tanks need careful maintenance
Medium KH (3-8 dKH): pH stable, suits most community tanks
High KH (10+ dKH): rock-stable alkaline pH, suits rift lake cichlids

This is the parameter most beginners ignore, and it is the one that causes most "sudden" pH crashes. A KH that slowly drops to zero over months lets pH nosedive one morning. Test KH monthly on any tank you care about.

The relationship between pH, GH, and KH

They are not independent. In most situations:

Hard water = high GH + high KH + high pH (they move together)
Soft water = low GH + low KH + low pH

When you lower pH by adding RO water, you lower GH and KH too. When you raise pH with crushed coral, you also raise GH and KH. This is why "pH adjusters" that only target pH are useless — they leave KH unchanged, so the buffer still wins, and the pH swings back to where it was.

UK regional tap water map

Know your water and your fishkeeping life becomes easier. UK hardness varies dramatically by geology^[4].

Regional overview

Region	Typical GH	Typical pH	Chlorine type	Naturally suits
London / SE England	17-22 dGH	7.4-8.0	Often chloramine	Livebearers, rift cichlids, snails
Essex / Kent	18-24 dGH	7.5-8.0	Chloramine	Hard-water specialists
East Anglia	14-20 dGH	7.2-7.8	Chlorine/chloramine	Community fish
Midlands	8-15 dGH	7.0-7.5	Chlorine	Most species, very flexible
South West	5-12 dGH	6.8-7.4	Chlorine	Community tropicals
Wales	4-10 dGH	6.8-7.3	Chlorine	Tetras, rasboras, apistos
NW England	4-10 dGH	6.8-7.3	Chlorine	Soft-water community
Yorkshire/NE	6-14 dGH	7.0-7.5	Chlorine	Flexible
Pennines	2-5 dGH	6.5-7.0	Chlorine	Soft-water specialists
Scotland	1-6 dGH	6.2-7.0	Chlorine	Discus, wild tetras, crystal shrimp
Northern Ireland	4-10 dGH	6.8-7.3	Chlorine	Community

Finding your exact water

Water company website — every UK water company publishes a postcode lookup. Search "[water company name] hardness postcode". Results show GH in dGH or ppm, KH, and treatment type
Test yourself — liquid kits give exact numbers. Fill a clean sample, test GH/KH/pH. Keep results logged over a few months to see any variation
Check your kettle — limescale in your kettle is a rough-and-ready hardness indicator. Heavy scale = hard water

UK note

Chloramine alert for London/SE keepers: Thames Water and some neighbouring utilities increasingly use chloramine rather than chlorine. Chloramine does not gas off from standing water and requires a dechlorinator specifically rated for it (Seachem Prime is — many cheaper alternatives are not)^[6]. Check your water conditioner's label.

How to test water properly

Testing is non-negotiable. A £25 kit pays for itself the first time it catches a problem early.

Kit recommendations

API Freshwater Master Kit — the gold standard for beginners and intermediates. Tests pH, ammonia, nitrite, nitrate^[5]
Salifert, JBL, Red Sea — more precise, pricier, professional kits
GH and KH add-ons — £6 each, every serious keeper owns them
TDS meter — £15 from Amazon. Not essential but useful for RO users and shrimp keepers
Test strips — avoid. Inconsistent, expire fast, and frequently wrong

What to test and how often

Stage	Test	Frequency
Cycling	Ammonia, nitrite, nitrate	Daily
First month stocked	All five (add pH)	Weekly
Established tank	All five	Monthly
Before water change	Nitrate	Every water change
Troubleshooting	All + GH, KH	As needed

Reading the tests properly

Compare against the card in natural light — yellow bathroom lights make blues look green
Shake test tubes exactly as instructed — undershaking gives false readings
Expiry dates matter — nitrate reagent especially degrades after 2 years
Between-tube colours are real — 5 ppm nitrate sits between the 0 ppm and 10 ppm squares

Record your results

A simple notebook or spreadsheet column per test. Log date, parameter, reading, anything notable (fish behaviour, recent changes). Patterns emerge only with data. A nitrate creeping from 10 to 20 to 30 over three months tells you your water change volume is too small — you would never notice that from a single test.

Adjusting parameters — when and how

Most of the time, do not. But sometimes you must.

When adjustment is justified

You keep soft-water species and your tap is hard (and vice versa)
You are breeding specialist fish that require specific water
Your nitrate straight from the tap is already above 20 ppm

When it is not

Chasing "perfect" numbers for community fish
Trying to match a care guide's exact pH
Fixing a problem you do not actually have

Lowering pH and hardness

The right way: RO water mixing.

Buy or install an RO unit (£80-£250 for a home unit)
Mix RO with tap water at a ratio that gives your target GH. Typical community-soft mix: 50% RO, 50% tap in medium-hard areas; 70% RO, 30% tap in hard London
Always remineralise RO-only water — pure RO is zero-mineral and shock-dangerous. Use Salty Shrimp GH/KH+, Tropic Marin Re-Mineral, or Equilibrium
Mix before adding to the tank — never mix in the tank

Biological methods stack on top:

Indian almond leaves — soft tannins, gentle pH drop, anti-fungal. Add 1 leaf per 20 litres, replace monthly
Alder cones — similar effect, more acid
Driftwood — gradual tannin release for months after adding
Peat filter media — older-school method, works but messes with water colour

Raising pH and hardness

Crushed coral in the filter — 100-200g in a mesh bag dissolves slowly, raises both GH/KH/pH over weeks. Replace yearly
Aragonite sand substrate — same effect, permanent
Limestone rocks in decor — useful and natural-looking for rift cichlid tanks
Seachem Equilibrium or Tropic Marin GH minerals — add with water changes for precise dosing

What to avoid

pH Up / pH Down bottled products — cause swings without changing KH. Stress fish more than the original problem
Baking soda — raises KH/pH crudely, hard to control
Phosphate-based buffers — fuel algae blooms

The nitrogen cycle

Everything about tank biology runs through this^[3]. Covered in full in our first tropical tank guide — here I will focus on ongoing management in a cycled tank.

Ammonia (NH3/NH4+)

Produced by fish, decaying food, and plant die-off. Toxic at any measurable concentration. Target: 0 ppm, always.

If ammonia appears in an established tank, something changed: dead fish hidden, overfeeding, filter died, medication killed the biofilter, or ammonia in source water. 50% water change immediately, diagnose root cause.

Nitrite (NO2-)

Produced by Nitrosomonas bacteria processing ammonia. Also toxic — prevents oxygen binding in fish blood. Target: 0 ppm, always.

Usually only seen during cycling or after a cycle crash.

Nitrate (NO3-)

End product of the cycle. Relatively non-toxic at low levels. Target: under 20 ppm ideally, under 40 ppm acceptable, above 40 ppm causes chronic health issues.

Controlled by water changes and live plants. Heavily planted tanks often run 5-10 ppm nitrate indefinitely. Fish-only unplanted tanks accumulate nitrate faster and need larger/more frequent water changes.

Cycle crashes

A stable biofilter can crash if:

Filter runs dry for more than 30-60 minutes
Filter media is replaced all at once
Medications (especially antibiotics) kill the bacteria
Sudden temperature drop
Power cut for 6+ hours

Signs: ammonia appears, fish gasp, behaviour becomes erratic. Treat as new cycle — daily 50% water changes to hold ammonia under 0.5 ppm while the colony rebuilds over 1-2 weeks. Bottled bacteria (Dr Tim's) help.

Temperature

Temperature determines metabolism, oxygen solubility, and disease resistance. Too high or too low is stressful; swings are worse than steady off-target values.

Ideal ranges by species type

Species group	Temperature	Notes
Coldwater (goldfish)	15-22 C	Not tropical — separate tank needed
Coolwater tropicals (celestial pearl danios, kuhli loaches)	20-24 C	Lower end of tropical
Standard community	22-26 C	Default tropical setup
Warm-water specialists (discus, cardinal tetras)	26-30 C	Higher end

UK winter considerations

Unheated rooms in UK homes drop to 12-18 C overnight in winter. Every tropical tank needs a heater. Rule of thumb: 1 watt per litre, adjustable thermostat, separate digital thermometer to verify.

Summer considerations

UK summers increasingly hit 30+ C. Tank temperatures can follow. Strategies:

Turn off tank lights during heatwaves
Float ice-filled water bottles in the tank (not ice cubes directly — shocks fish)
Point a fan across the water surface — evaporative cooling drops temperature 2-4 C
Increase surface agitation to boost oxygen

TDS and conductivity

Total Dissolved Solids measures the total dissolved mineral content of water. Measured in ppm with a £15 conductivity meter.

Pure RO water: 0-5 ppm
Typical UK tap: 150-500 ppm (low in Scotland, high in SE)
Typical aquarium: 200-600 ppm (higher than source due to evaporation and feeding)

For most fish, absolute TDS does not matter within reasonable ranges. For shrimp keepers — especially Caridina — TDS stability matters more than GH/KH testing, because TDS catches subtle shifts the other tests miss. Keep Neocaridina shrimp tanks in a 150-250 ppm range, Caridina in 100-180 ppm.

Why TDS rises over time

Water evaporates but dissolved minerals do not. A tank on day 7 after a water change has higher TDS than day 1. Topping up only with RO water (to replace evaporation) and using full water changes (rather than top-ups) to refresh minerals keeps TDS stable.

Chlorine and chloramine removal

UK tap water is treated with chlorine or chloramine to kill pathogens^[6]. Both kill beneficial bacteria and damage fish gills.

Chlorine

Traditional disinfectant. Evaporates from standing water in 24-48 hours. A standard dechlorinator instantly neutralises it.

Chloramine

Increasingly common in the UK (Thames, Anglian, parts of the SE). A stable combination of chlorine and ammonia that does not evaporate and needs chemical neutralisation. Many older water conditioners handle only chlorine — check your bottle.

Recommended conditioners

Seachem Prime (£12 for a long-lasting bottle) — handles chlorine, chloramine, plus detoxifies ammonia/nitrite for up to 48 hours. My default
API Tap Water Conditioner — chlorine only, cheaper
Tetra AquaSafe — chlorine and chloramine, includes aloe-based slime coat support

The standing-water myth

"If I leave tap water out for 24 hours, chlorine evaporates." True for chlorine, false for chloramine. If your water company uses chloramine, standing water does not become safe. Always dechlorinate.

Why stable beats perfect — the single biggest beginner myth

This is the point I most want to hammer home. More fish die from parameter swings than from off-target-but-stable parameters, by a wide margin.

The biology: fish regulate their internal salts and fluids through osmosis. Their bodies are adapted to the water chemistry of their environment. Sudden changes — especially in GH, TDS, and pH — force the fish's body to rapidly readjust osmotic balance, which is metabolically expensive and physiologically stressful.

The implication: a tank held at pH 7.6 for five years straight is healthier for a neon tetra ("prefers" 6.5) than a tank swinging between 6.2 and 6.8 every week because the owner is dosing pH down with each water change.

The rule:

Know your tap water
Choose fish that tolerate it
Change water consistently (same volume, same parameters, same temperature, same intervals)
Do not chase numbers

The only people who should be routinely adjusting parameters are:

Breeders replicating specific spawning conditions
Soft-water specialists keeping discus in London
Rift cichlid keepers in Scotland
Shrimp keepers running precision Caridina setups

Everyone else: stop dosing. Start observing.

Putting it together — sample parameter targets

Standard community tank (most UK keepers)

Temperature: 24 C
pH: 7.0-7.4 (whatever your tap runs at, stable)
GH: 6-12 dGH
KH: 3-8 dKH
Ammonia: 0
Nitrite: 0
Nitrate: under 20 ppm
TDS: 200-350 ppm

Stocking: neon tetras, corydoras, harlequin rasboras, cherry shrimp, nerite snails, honey gouramis

Soft-water blackwater tank

Temperature: 26 C
pH: 5.8-6.4
GH: 2-4 dGH
KH: 0-2 dKH
Tannin-stained water
TDS: 80-150 ppm

Stocking: wild-form cardinal tetras, ember tetras, apistogramma, discus

Hard-water livebearer tank

Temperature: 24-26 C
pH: 7.8-8.2
GH: 15-20 dGH
KH: 8-12 dKH
TDS: 400-600 ppm

Stocking: guppies, platies, mollies, endler guppies, nerite snails

Shrimp-only Neocaridina tank

Temperature: 22 C
pH: 7.0-7.6
GH: 8-12 dGH
KH: 3-6 dKH
TDS: 200-300 ppm
Zero copper

Stocking: cherry shrimp, nerite snails

Emergency troubleshooting

Ammonia above 0.25 ppm

Immediate 50% water change. Match temperature. Reduce or skip feeding for 2-3 days. Test daily until zero. If it returns, check for dead fish, filter failure, overfeeding, or overstocking.

Nitrite above 0.25 ppm

Same as ammonia plus consider raising KH slightly — nitrite is more toxic in soft acidic water.

pH suddenly crashed

Test KH. If near zero, do large water changes with your higher-KH tap water to rebuild the buffer. Add crushed coral in filter long-term. Never try to raise pH quickly — that causes a second swing.

All fish gasping at surface

Oxygen crisis. Increase surface agitation (turn up filter outlet, add airstone), check temperature is not above 28 C, do 50% water change, reduce stocking if overloaded.

Cloudy water in new tank

Bacterial bloom — normal during cycling, clears within 1-2 weeks. Do nothing.

Green water

Algae in suspension — too much light or nutrients. Reduce light duration to 6 hours, blackout the tank for 3 days (cover completely), then return to shorter photoperiod.

Why buy from us

We ship test kits, dechlorinators, and water treatment alongside our live fish orders. Everything packed together for single-delivery convenience. Genuine UK stock, delivery across the mainland.

Our team keeps UK-typical tanks ourselves — if you want straight advice on your postcode's water, drop us an email before you stock. We know what thrives in London hard water versus Scottish soft because we run tanks in both.

Answers to common questions

What is the best pH for aquarium fish?

There is no single best pH — it depends on the species. For most community tropical fish, anything stable between 6.5 and 8.0 is fine. Match your fish to your tap water's pH rather than chasing a number.

How do I test my aquarium water?

Liquid test kits — API Freshwater Master Kit is the standard. Test ammonia, nitrite, nitrate, and pH at minimum. Add GH and KH test bottles for full coverage. Never use strips.

How often should I change water?

Weekly, 20-25% of tank volume, for most tanks. Heavily stocked or unplanted tanks may need more. Lightly stocked, heavily planted tanks can go fortnightly. Always use dechlorinated water at tank temperature.

Can fish live in tap water?

Yes, once dechlorinated. UK tap water is safe for most community species. Only soft-water specialists in hard-water regions need RO mixing. See the UK water map above for your area.

What is the best water conditioner?

Seachem Prime is the industry default — handles chlorine, chloramine, and detoxifies ammonia/nitrite. Lasts a long time (5ml treats 200 litres). Cheaper alternatives work for chlorine-only areas.

Why is my pH high?

UK tap water in hard-water regions naturally runs pH 7.5-8.0. If that is your tap, that is your tank. Choose hard-water-friendly species or mix with RO water to soften.

Do I need to use RO water?

Only if you keep soft-water specialists (discus, wild tetras, crystal shrimp) in a hard-water region. For normal community fish, tap water with dechlorinator is better — pure RO has zero minerals and needs remineralising.

What is ammonia poisoning?

Ammonia burns gill tissue and causes red/bloody gills, rapid breathing, lethargy, and death. Always zero in a cycled tank. If above zero, large water change and find the cause.

Why is my tank cloudy?

New tank: bacterial bloom, normal, clears in 1-2 weeks. Established tank: overfeeding, overstocking, or filter failure. Green cloudy = algae (light/nutrient issue).

Is bottled water OK for aquariums?

Mineral water is often too hard or variable. Distilled/deionised water is too pure. Tap water with dechlorinator is better than either for most tanks. RO water with remineraliser is the only bottled-type water that really works, and it is cheaper to make yourself.

What if my tank is in direct sunlight?

Move it. Sunlight causes algae blooms and unpredictable temperature spikes. A consistent artificial light on a timer is always better.

How do I lower nitrate?

Larger or more frequent water changes, live plants (especially fast-growers like hornwort or pothos), reduce feeding, consider a nitrate-removing filter media. Check source water — if your tap is 20+ ppm nitrate, you will never get below that without RO mixing.

What does KH actually do?

It prevents pH swings. A KH of 0 means pH can crash at any moment as acids build up. A KH of 5+ holds pH steady against normal biological acid production. It is the most underappreciated parameter in the hobby.

How do I know my cycle is complete?

Dose 2-4 ppm ammonia. Wait 24 hours. Test ammonia and nitrite — both should read zero. Nitrate should be elevated. That is a cycled tank. See our first tropical tank guide for the full fishless cycling procedure.

At a glance

The three parameters that matter most: temperature, ammonia/nitrite, pH stability
Test kit: liquid (API Master), never strips
UK water: soft in Scotland/NW, hard in London/SE, variable everywhere else
Dechlorinator: every water change, every time, no exceptions
Rule of thumb: match your fish to your water, not your water to your fish

Mistake I've made

Good water chemistry means clear water, healthy fish, and zero drama

The three measurements that actually matter

Every fishkeeping forum argument can be distilled to confusion about which parameters matter. Here is my priority list, built over years of seeing what kills fish and what does not.