In a community tank a fish is not only lost to a nasty neighbour. Water that does not suit it, or a mouth it fits into, is enough. Every pair can be checked against the fish's parameters, without relying on someone else's "they live together just fine".

Fish get along when they share the same water, neither can swallow the other, their temperaments do not clash, there is enough room and the habits of the species do not hurt the neighbour. That makes five questions. If the answer to any of them is "no" or "I don't know", do not take the pair.

Question one: does the same water suit both

Compare four numbers for both fish: temperature, pH, hardness (dGH) and salinity. The ranges must overlap, and the water in the aquarium must lie inside the overlap. What pH and hardness are is explained in the article Water chemistry.

An overlap can also shrink to a single point: the guppy and the neon tetra share only pH 7 and hardness 10 dGH. Such water would have to be held at the very edge of both fish's ranges, and the compatibility page marks such pairs "Caution".

Question two: will one fish fit in the other's mouth

Compare adult sizes. Look at the length of the adult fish in an aquarium, not at the size of the juvenile in the shop. On the species page there is a "Toward small fish" line next to it: "predator", "not a predator" or "a risk to small fish". The last means that the species does not hunt by habit, but will swallow a fish that fits in its mouth.

  • Angelfish grows to 15 cm, and its diet is small fish and invertebrates. The neon tetra is 4 cm. It is not advised to put fish of the angelfish's size or smaller with it.
  • Oscar cichlid (Astronotus ocellatus) is 35 cm, and its diet includes small fish. Its tank mates are chosen only from sufficiently large and calm fish. Paired with the neon tetra it is rated "Not recommended".

Fish with this behaviour are collected in the Predators category. The goldfish is not in it, but its parameters say "Semi-peaceful (dangerous to smaller ones)", and it grows to 30 cm.

Question three: temperament

In the catalogue temperament is recorded as one of twelve wordings. The main ones: "Peaceful", "Semi-peaceful (aggression when space is limited)", "Territorial (often aggressive)", "Predator", "Peaceful, but may bite fins".

"Semi-peaceful" reads as "peaceful on conditions": a spacious aquarium, hiding places, suitable neighbours. "Territorial" means the fish defends its patch against everyone.

  • Siamese fighting fish (Betta splendens) is "Territorial". The male attacks anything that even remotely resembles a rival. Two males must never be kept together, and tank mates are most often not advised. Paired with the angelfish or the guppy it is rated "Not recommended".
  • Guppy and platy are both "Peaceful", and the pair is rated "Compatible".

Question four: who swims where, and is there enough room

For each species a swimming zone is given: near the bottom, the lower layer, the middle, near the surface. Fish from different zones meet less often, but this is no guarantee (see below). So the neon tetra stays in the middle layer, the bronze corydoras (Corydoras aeneus) near the bottom, and the guppy and platy near the surface.

The neon tetra and the bronze corydoras also fit on the other questions: both are peaceful, and the water is shared (21–25 °C, pH 6–7, hardness 5–10 dGH). The pair is rated "Compatible".

The recommended volume is given for a group of that species, not for one fish. For the neon tetra it is 54 litres, for the bronze corydoras 70 litres. For two species take no less than the larger of the numbers, and keep shoaling fish in a shoal. Why a small volume is harder than a large one is in the article A 10–20 litre aquarium.

Question five: what is known about the species' habits

Open the pages of both species and read the "Behavior and Compatibility" section. It holds what the numbers do not.

  • Sumatran barb (Puntigrus tetrazona) is "Peaceful, but may bite fins" in the parameters. The description says more: it nips long fins and barbels, and gouramis, goldfish and fish with veil fins are at risk. Paired with the goldfish or the angelfish it is rated "Not recommended".
  • It is also a shoaling fish: at least 8–10 are recommended, and single barbs may show aggression to their neighbours.
  • Neon tetra without a shoal of 8–10 loses vigour and colour, and its life is shorter.

What does not work

Peaceful species are compatible automatically. The neon tetra and the molly are peaceful, but their water differs. The Sumatran barb is recorded as "peaceful" with a reservation about fins, and that reservation matters more.

Fish of the same size get along. The fighting fish and the Sumatran barb are both 7 cm, and the pair is still rated "Not recommended": the barb nips fins, and the fighting fish attacks a rival. The angelfish, on the contrary, is not advised tank mates "smaller or equal in size".

Different layers will not cross paths. Catfish live near the bottom, the angelfish in the middle layer. But during spawning the angelfish guards a patch and attacks anyone who comes close, so the pair is rated "Caution".

The size in the shop is the size for life. A juvenile oscar is promised 35 cm, and it needs a 500-litre aquarium.

What to check before buying

  1. Write down the temperature, pH, hardness and salinity of both fish. Find the overlap.
  2. Compare adult sizes and the "Toward small fish" line.
  3. Read the temperament of both. "Territorial" and "may bite fins" each need a separate answer.
  4. Compare the swimming zones and take a volume no less than the larger of the two.
  5. Read "Behavior and Compatibility" for both species, including shoal size.
  6. Open the compatibility page: pick a fish and a second one for the pair. The statuses are "Compatible", "Caution", "Not recommended"; if there is no behaviour assessment, "Insufficient data".
  7. For species not on the list, the calculator will assess the pair. It compares water, size and temperament.
  8. Stock the fish in batches, as the article Starting an aquarium and the nitrogen cycle advises.

The limit of any table

The ratings for 510 popular species were compiled by hand from the described behaviour of the fish. For the rest, a calculation based on water, size and temperament is used. Both methods give a generalised answer.

But fish of the same species differ in character. Of the fighting fish it is written that individual fish tolerate neighbours, but that is the exception, not the rule. "Compatible" is not a guarantee, and "Not recommended" is not a sentence. A rating helps you avoid an obvious mistake; after that, watching your own aquarium decides.