Aquarium Water Parameters: Complete UK Guide (2026)

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.

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 fish examples (discus, wild-caught cardinal tetras): 5.5-6.8
• Freshwater shrimp: use taxon-specific water checks, because “Caridina” does not define one pH range.
• 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

Finding your exact water

1. 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
2. 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
3. Check your kettle — limescale in your kettle is a rough-and-ready hardness indicator. Heavy scale = hard water

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

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.

1. Buy or install an RO unit (£80-£250 for a home unit)
2. 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
3. Always remineralise RO-only water — pure RO is zero-mineral and shock-dangerous. Use Salty Shrimp GH/KH+, Tropic Marin Re-Mineral, or Equilibrium
4. 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

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:

1. Know your tap water
2. Choose fish that tolerate it
3. Change water consistently (same volume, same parameters, same temperature, same intervals)
4. 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.