By Hannah Nielsen
Which tetra is right for you? Start here
If you've searched "tetra", you're almost certainly standing in front of a tank — or planning one — and trying to work out which of the dozens of small, colourful fish on offer is the right one. That's the decision this page exists to make for you, and the honest answer is: it depends on your tap water and your tank size more than on which fish looks prettiest in the shop photo.
I'm Hannah, the tetra and rasbora specialist here. I've spent the best part of a decade photographing planted community tanks, and the question I get asked more than any other is some version of "I want tetras — which ones?" This guide is the answer I'd give a customer standing at our tanks: not a list of every tetra on earth, but a clear comparison of the species we actually stock and keep, grouped by the two things that decide success — your water hardness and your tank volume.
Tetras are small shoaling fish, the vast majority from South America (family Characidae), with a few — like the congo tetra — from Africa (family Alestidae) [1]. They're peaceful, hardy, affordable, and a school of them drifting through planted water is one of the loveliest sights in the hobby. They are also, without exception, fish you buy in groups — never ones and twos.
The neon tetra (Paracheirodon innesi) — the fish that launched a thousand community tanks. That iridescent blue line is a structural colour, not a pigment, so it flares brightest under good LED lighting. Photo: Tropical Fish Co.
Five things about tetras most UK guides never mention
- The neon tetra's blue stripe isn't a pigment — it's an optical trick. The shimmering blue line is structural colour produced by light bouncing off stacked guanine crystals (iridophores), which is why it looks like it switches off at night and blazes under daylight LEDs [1].
- Cardinal and neon tetras are different fish, often confused. The neon's red runs only along the back half of the body; the cardinal's red runs the full length, nose to tail. Cardinals are also larger and want softer, more acidic water — pH 4.0–6.0 in the wild [2] versus the neon's wider range.
- Tetras genuinely prefer bigger shoals — it's measurable. Peer-reviewed work on shoaling fish shows individuals actively choose larger groups because a bigger shoal dilutes each fish's predation risk [3]. "Six minimum, ten-plus ideal" isn't folklore — it's behavioural science.
- Not all tetras come from the Amazon. The striking congo tetra is African, from the Congo River basin, and grows to 8 cm — far larger than the nano South American species and needing a 150 L tank to suit it.
- Most UK tap water is the wrong water for cardinals. Thames Water and most of the south-east supplies hard water, drawn through chalk and limestone [6]. That's fine for ember, black neon and lemon tetras, but cardinals and rummy-nose will sulk in it — which is exactly why your shop choice should follow your tap, not the other way round.
Popular tetras compared
Every number below is the real care spec we keep on file for the fish swimming in our tanks. Use it to shortlist by your water (the hardness column) and your tank (the minimum-volume column) first — then pick on looks.
| Species | Colour | Adult size | Min tank | Hardness tolerance | Temperament / fin-nipping | Beginner rating |
|---|---|---|---|---|---|---|
| Neon Tetra | Blue + red stripe | 4 cm | 40 L | Soft–moderate (1–10 dGH) | Peaceful, no nipping | ★★★★★ |
| Ember Tetra | Glowing orange | 2 cm | 30 L | Flexible (2–10 dGH) | Peaceful, no nipping | ★★★★★ |
| Black Neon Tetra | Black + silver stripe | 4 cm | 40 L | Hard-water OK (3–15 dGH) | Peaceful, no nipping | ★★★★★ |
| Lemon Tetra | Lemon yellow, red eye | 4.5 cm | 60 L | Hard-water OK (4–15 dGH) | Peaceful (mild hierarchy) | ★★★★☆ |
| Cardinal Tetra | Full red + blue | 5 cm | 60 L | Soft only (1–8 dGH) | Peaceful, no nipping | ★★★☆☆ |
| Rummy-Nose Tetra | Silver body, red nose | 5 cm | 80 L | Soft (2–8 dGH) | Peaceful, tight shoaler | ★★★☆☆ |
| Flame Tetra | Orange-red flush | 4 cm | 60 L | Hard-water OK (3–15 dGH) | Peaceful (mild nipper if under-grouped) | ★★★★☆ |
| Black Phantom Tetra | Smoky grey, black flank | 4.5 cm | 80 L | Adaptable (5–15 dGH) | Mostly peaceful, can spar | ★★★★☆ |
| Serpae Tetra | Deep blood-red | 4 cm | 60 L | Adaptable | Known fin-nipper — group 8+ | ★★★☆☆ |
| Congo Tetra | Iridescent rainbow | 8 cm | 150 L | Adaptable (3–12 dGH) | Peaceful but large + boisterous | ★★★☆☆ |
If your tap is hard (most of England) and your tank is under 60 L, the rows to look at are ember, black neon and neon tetras. If you have soft water or RO, the cardinal and rummy-nose rows open up.
The schooling rule, tank size, and water — in that order
Three things decide whether your tetras thrive, and they matter in this exact order.
1. School size — the rule that overrides everything
Buy a proper group. Six is the floor; ten or more is the target. This is not a sales line — it's how these fish are wired. In a shoal that's too small, tetras hide, their colour fades, and the bolder species redirect their natural sparring onto each other's fins or their tankmates' [3]. A single big school of one species always beats a scattering of several. Twelve neon tetras look like a display; three neons plus three cardinals plus three embers look like a mistake.
2. Tank size — match the most demanding species
Stock to the fussiest fish on your list:
- 30 L planted nano — 10 ember tetras (the only true nano tetra)
- 40 L planted — 10–12 neon or black neon tetras
- 60 L planted — 12 lemon, flame or cardinal tetras
- 80 L planted — 12 rummy-nose or black phantom tetras (room to swim)
- 120 L+ planted — mixed Amazon display: neon + cardinal + ember shoals
- 150 L+ — a shoal of 8 congo tetras (large, active, needs length)
3. Water — follow your tap, don't fight it
Most tetras want soft, slightly acidic water (pH 5.5–7.5) [1], but tolerance varies enormously. Test your tap hardness first. If it's hard — which covers most of England [6] — build around ember, black neon, lemon or flame tetras, which hold colour in hard water. Save cardinals and rummy-nose, which genuinely need soft water under 8–10 dGH [2], for a tank you can soften with RO or rainwater.
If you only remember one thing from this page: a big shoal of a single tetra species is calmer, healthier and more striking than a "pick and mix" of several. Each tetra species only shoals with its own kind, so six each of four species gives you four anxious mini-groups. Twelve of one gives you a school. Buy depth in one species before you add a second.
Best tetras for a community tank
Black neon tetras (Hyphessobrycon herbertaxelrodi) shoaling — my first pick for a UK beginner with hard tap water. The horizontal black-and-silver stripe reads beautifully against green plants, and they tolerate up to 15 dGH. Photo: Tropical Fish Co.
For a peaceful planted community, these are the species I reach for first — each links to its full care guide and, where we've written one, its own dedicated buying guide:
- Neon tetra care guide — the classic beginner shoaler; peaceful, cheap, brilliant in a planted 40 L.
- Cardinal tetra care guide — the soft-water showpiece; bigger and redder than the neon. See the full cardinal tetra UK buying guide for water specifics.
- Ember tetra care guide — the hard-water hero and the only true nano tetra; our ember tetra UK buying guide covers nano setups.
- Harlequin rasbora care guide — not a tetra, but the classic tetra companion: same water, same gentle temperament, complementary colour.
Pair any of these with corydoras on the substrate, a honey gourami as a centrepiece, and a clean-up crew of shrimp, and you have a textbook UK community tank. For the broader shortlist, our tetras hub lists every tetra in stock this week.
A note on that last one: the glowlight rasbora (Rasbora hengeli) is a true rasbora, not a tetra — it belongs to the carp family, Cyprinidae, not Characidae. We've included it because it shoals, sizes and behaves exactly like a small tetra and slots into the same community, but it's only fair to name it accurately.
Tank mates that genuinely work with tetras
Tetras are peaceful mid-water fish, so their best companions occupy other zones of the tank or share their gentle temperament:
- Corydoras catfish — the perfect substrate companion; peaceful, also shoaling, and they work the bottom while tetras hold the middle.
- Harlequin & lambchop rasboras — same water, same temperament; a rasbora shoal alongside a tetra shoal is a classic layered display.
- Honey gouramis — a single pair makes a gentle centrepiece in a 40 L+ without threatening the tetras.
- Otocinclus — soft-water algae specialists that suit the planted, slightly-acidic setup tetras prefer.
- Cherry & amano shrimp — completely safe with adult shrimp; tetras may pick off newborn shrimplets, which usefully controls a shrimp colony.
Avoid: anything over ~6 cm that could eat a tetra; angelfish (which eat neon-sized tetras once mature); and pairing fin-nippers like serpae tetras with long-finned fish such as bettas or fancy guppies [5].
When your tetras arrive — our UK delivery and acclimation protocol
Tetras ship well — their small body size means low ammonia output in the bag — but they're sensitive to sudden pH and temperature changes, so we acclimatise them gently. Our protocol:
- Dim the room and open the box carefully. Bright light on arrival stresses fish that have travelled in the dark.
- Float the sealed bag for 15–20 minutes to equalise temperature.
- Drip-acclimate for about 30 minutes at 1–2 drops per second. Tetras tolerate this pace well; for soft-water sensitive species like cardinals and rummy-nose, extend toward 40–45 minutes [7].
- Net the fish into the tank — never pour the bag water in. Bag water carries waste you don't want in your display.
- Lights off for two hours so the shoal can settle without crowding.
- No feeding for 24 hours. A fasted, settled fish is a calm fish.
Every order travels with our live arrival guarantee. If any fish arrive in poor condition, photograph the unopened bag within two hours of delivery and we'll refund or replace.
Where you can, run new tetras through a two-week quarantine tank before adding them to an established display. Neon tetra disease (a Pleistophora parasite) spreads in crowded retail systems and has no cure once a fish is symptomatic — look for fading colour, a lumpy or curved spine, and a fish that drops out of the shoal. Quarantine protects the school you've already built.
Ready for more?
You've got the shortlist; here's where to go next.
- Shop: the tetras hub for every tetra in stock, or the broader tropical fish for sale UK page for the full community-tank range.
- Learn: the neon tetra care guide and cardinal tetra care guide go deeper on the two species most people start with.
- Compare: the cardinal tetra UK buying guide for the soft-water showpiece, and the ember tetra UK buying guide for the hard-water nano hero — the two ends of the tetra spectrum.
The References block renders automatically below from the page's sources. Every claim above is grounded in FishBase, Seriously Fish, peer-reviewed shoaling research, Thames Water's hardness data, and the care specs of the fish in our own tanks.
