Freshwater fish compatibility

Updated on August 26, 2026 · ~7 min read

Compatibility is decided on four checks, in this order: the water type must match, the species must fit the tank's volume, the temperaments must not clash and the adult sizes must not be too far apart. One failed check is enough to rule the combination out.

"Do these two fish get along?" is the question people buy badly with. Not because there is no answer, but because they nearly always look for it in a table of pairs — and the pairs number in the thousands, while the criteria that decide them are four.

This page explains the four checks. They are the same ones the app applies before an inhabitant is added, and they take thirty seconds with the species' details in hand.

1. The water type must match

This is the only check that gives a flat no: a saltwater fish does not go in a freshwater tank, and the other way round. It sounds obvious, and yet it is the most common mistake at the fish counter — many brackish species (the dwarf pufferfish, mollies, several gobies) are sold as "tropical freshwater" and survive for months in fresh water, but they do not live their lives.

In the same family of errors: a very soft-water fish put in very hard water, or the reverse. It does not kill within a week; it shortens the life and makes breeding difficult. It is worth looking at the GH and KH of your own tap before choosing inhabitants: it is far easier to pick fish suited to your water than to correct the water for years.

Water	Who lives in it
Soft, 2-6 dGH, pH 6.0-7.0	discus, neon and cardinal tetras, many characins, Caridina
Medium, 6-12 dGH, pH 6.5-7.5	most community fish, Neocaridina
Hard, 8-15 dGH, pH 7.8-8.5	Malawi and Tanganyika cichlids, many livebearers

2. The species must fit the tank's volume

Every species has a recommended minimum volume, which depends on adult size, on how much it swims and on how territorial it is. It is a hard constraint, and it comes before the load calculation.

Some references from the catalogue:

Species	Adult size	Minimum volume
Chili rasbora	2 cm	40 L (11 gal)
Neon tetra	3 cm	60 L (16 gal)
Guppy	4 cm	40 L (11 gal)
Betta	7 cm	20 L (5 gal)
Bronze Corydoras	7 cm	75 L (20 gal)
Tiger barb	7 cm	90 L (24 gal)
Ram cichlid	5 cm	80 L (21 gal)
Bristlenose pleco	13 cm	100 L (26 gal)
Siamese algae eater	14 cm	100 L (26 gal)
Goldfish	25 cm	100 L (26 gal)
Discus	20 cm	250 L (66 gal)
Clown loach	30 cm	300 L (79 gal)
Oscar	35 cm	300 L (79 gal)

⚠️ The size that counts is the adult one, not that of the specimen in the shop tank. A clown loach is sold at 4 cm and reaches 30: the 60-litre tank it went into will not be enough in two years, and nobody connects the death then with the purchase now.

3. The temperaments must not clash

Three categories, and the practical rule is simple: an aggressive fish with peaceful ones is a problem, in both directions.

Peaceful — neon tetras, rasboras, Corydoras, guppies, platies, otocinclus. They live together and with most of the calmer semi-aggressive species.

Semi-aggressive — tiger barb, blue gourami, apistogramma, serpae tetra, red-tailed black shark. Territorial in certain phases, or fin nippers. They live with animals of their own size, not with slow long-finned fish.

Aggressive — betta, Malawi cichlids (mbuna), oscar, green terror, Jack Dempsey, convict cichlid. They belong in tanks designed for them, and in many cases alone or with their own species.

Two cases worth knowing because they come up constantly:

• The betta. It is aggressive, with one particularity: it attacks above all what resembles it (long fins, showy colours). A male with another male is out of the question; with male guppies it is nearly always a disaster; with a shoal of small cyprinids in a big enough tank it can work.
• Malawi cichlids. They are not "aggressive fish to be kept in small numbers": they are aggressive and they should be kept in many, in tanks of 200-250 litres (53-66 gal) and up, because the aggression is diluted across the group. This is the case in which "fewer animals" makes things worse.

4. The adult sizes must not be too far apart

This is not a rule of good taste: it is that in an aquarium a fish eats whatever fits in its mouth, even a species described as peaceful. An adult angelfish and neon tetras live together badly not out of aggression but because of size. The rule of thumb: if one fish is more than twice the length of another, it is worth checking what it eats in the wild.

And the reverse: very small fish in a tank with large ones stay hidden, eat little and live stressed even when they are not preyed on.

Three criteria that are not compatibility but decide it anyway

The shoal. Neon and cardinal tetras, rasboras, Corydoras, tetras and most small gregarious species belong in groups of at least 6-8. Three neons are not a small shoal: they are three stressed animals, and stress opens the door to disease. Better one large shoal than three species of three.

The zone of the tank. A tank works better if its inhabitants spread out: surface fish, mid-water fish and bottom fish. Three species that all live on the bottom compete for the same territory in a tank that looks half empty.

Temperature. Two species compatible in everything else but with temperature ranges that do not overlap cannot live together: goldfish (18-22 °C / 64-72 °F) and neon tetras (24-27 °C / 75-81 °F) are the classic example, and the tank they share is bad for both.

An example: a 100-litre (26 gal) community that works

• 10 neon tetras (3 cm, min 60 L) — mid-water shoal
• 6 bronze Corydoras (7 cm, min 75 L) — bottom group
• 1 ram cichlid or a pair (5 cm, min 80 L) — lower mid-water

Load: 30 + 42 + 10 = 82 cm against a capacity of about 67 cm → 122%, overstocked. Dropping two Corydoras brings it to 68 cm, that is 101%: still at the limit. That is the sum which explains why a 100-litre tank holds two small shoals and not three — and why all the "100 litre stocking lists" you find around are a little generous.

What to do when the check says "caution"

A warning is not a ban. The catalogue is a guide, not a rulebook: a quarantine tank, a juvenile specimen, a very large planted tank with places to hide can all make acceptable a combination that looks risky on paper. Anyone who knows why they are bending the rule may bend it.

What does not work is ignoring the warning without knowing which one it is. "Caution over volume" and "caution over temperament" call for two different decisions: the first is solved with a bigger tank a year from now, the second is solved only by choosing a different fish.

How Gurglee does it

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