Which Freshwater Fish Can Share a Tank, and Why

What decides whether two aquarium fish are compatible?

Aquarium fish compatibility is decided by four things: shared water, temperament, adult size and space. A pairing that fails any one of them fails as a whole, however well the other three line up. This is why a simple "peaceful" or "community fish" label in a shop is not enough on its own.

The four conditions work like this:

• Shared water. Both species need a temperature, pH and hardness range that overlaps. A goldfish kept at 59–70 °F (15–21 °C) and a betta fish that needs 72–86 °F (22–30 °C) have no temperature in common at all, so one of them is always outside its range.
• Temperament. A fish that chases, nips or fights will stress a calm tank mate even when the water suits both. Male bettas, for example, may attack long-finned fish they mistake for rivals, according to Seriously Fish.
• Adult size and predation. Fish eat what fits in their mouths. An angelfish grows to 6 in (15.2 cm), and a neon tetra reaches only 1.2 in (3 cm), so the tetra is food once the angelfish is grown.
• Space. The tank has to suit the larger species' minimum and leave room for every school at its proper size.

Swim level matters too, but less: two species that use different layers of the tank compete less for room, while two bottom dwellers share one floor.

How does AquaCompat turn published data into a verdict?

AquaCompat turns published data into a verdict by applying one fixed formula to the two species' fact files. A fact file is the structured record for one species: its temperature, pH and hardness ranges, adult size, minimum tank, school size, temperament and diet, each value tied to the sources it came from. The formula weighs six factors, temperature overlap, temperament, size and predation risk, pH overlap, hardness overlap and swim level, and returns a score from 1 to 10.

A score of 8 or more reads as compatible, 5 to 7 as caution and below 5 as incompatible. Some problems override the score completely. If two species share no temperature range, or one eats fish the size of the other, the pair is marked incompatible whatever the arithmetic says. The full weighting, the list of deal-breakers and the method's limits are set out on the methodology page, so any verdict on the site can be traced back to its inputs.

Nothing on AquaCompat comes from our own tanks. The site compares published aquarium-care data from sources including Seriously Fish, FishBase, Aquarium Co-Op and Practical Fishkeeping, and where those sources disagree, the conflict is noted on the species page rather than hidden. The ranges used are aquarium-care ranges for farmed fish, which are usually wider than the water in the species' wild habitat.

Beyond single pairs, the free aquarium tools check a whole stocking list, the tank setups by size work through complete stocking plans, and the side-by-side species comparisons help when two similar species compete for the same spot. Who writes the site and how it is funded is explained on the about page.

Which fish pairs do people ask about most?

The pairs below are the ones searched most often in the US, ranked by search demand, with the verdict, score and shared temperature taken straight from the pair matrix. Each links to a full compatibility guide where one exists.

Why do some popular pairs fail?

Popular pairs fail for three repeating reasons: temperature, predation and temperament. Each of the incompatible results in the table above traces back to one of them.

Temperature: goldfish with tropical fish

Goldfish are the clearest temperature mismatch. Their aquarium-care range of 59–70 °F (15–21 °C) does not reach the betta's at all, and against the guppy's 68–82 °F (20–28 °C) the shared band is only 68–70 °F (20–21 °C), too narrow to hold steadily in a home tank. Goldfish also grow to 10 in (25.4 cm), large enough to swallow small tropical fish.

Predation: angelfish with small fish

Angelfish share water well with guppies, but an adult angelfish eats fish small enough to swallow, and a guppy at 2.4 in (6.1 cm) qualifies. The same applies to neon tetras. The size factor is judged at adult size, so a verdict stays incompatible even while both fish are still small.

Temperament: bettas and long fins

Betta fish with guppies scores caution rather than incompatible. The water overlaps well across 72–82 °F (22–28 °C), but male bettas may treat a long-finned male guppy as a rival. Whether it works depends on the individual betta, the tank size and how much cover the guppies have, which is exactly what a caution verdict is meant to flag.

Which species does AquaCompat cover?

AquaCompat covers 22 freshwater species chosen because they are the fish, shrimp and snails most often kept together in home community tanks. They range from the 1.5 in (3.8 cm) Cherry Shrimp to the Oscar, which reaches 14 in (35.6 cm), and from coldwater goldfish to discus that need 79–86 °F (26–30 °C). Each species has its own care data, and the freshwater aquarium species index lists all 22 side by side with their temperature, pH, adult size and minimum tank.

Covering a fixed set of species keeps the pair matrix complete: every species has a scored verdict against every other, so a tank mates list never leaves out a common candidate. The trade-off is that fish outside the 22, such as rarer tetras or dwarf cichlids, do not yet have verdicts.

Most fish of these kinds reach shops from farms rather than the wild. A 2019 Journal of Fish Biology review estimates that 90 % of ornamental fish trade volume is tropical freshwater fish and that about 90 % of those are captive bred, figures drawn from a 2013 trade analysis. The USDA 2023 Census of Aquaculture counted 70.4 million freshwater egg-layers and 23.4 million livebearers sold by US farms that year, most of them from Florida.

What should you do before adding any fish?

Before adding any fish, cycle the tank so that bacteria are in place to process fish waste. The nitrogen cycle is the process in which bacteria convert toxic ammonia from waste into nitrite and then into the far less harmful nitrate. A new tank has none of those bacteria, and fish added too early sit in rising ammonia, the problem known as new tank syndrome.

The guides below cover the setup order: how the cycle works, how to complete it without fish, and what a first small community tank can hold. Compatibility verdicts assume a cycled, stable tank; no pairing is safe in water that is still cycling.

Which equipment affects compatibility?

Heaters, filters and test kits affect compatibility more than any other equipment, because they decide whether the tank actually stays inside the shared range. A pair that overlaps by only a few degrees, such as Cardinal Tetra and Neon Tetra at 73–77 °F (23–25 °C), needs a heater that holds temperature closely. Fish with very different flow preferences need a filter that can be baffled or sized down, and a test kit is the only way to know the pH and hardness you are working with.

The equipment guides compare products on their published specifications, matched to tank size, rather than on ratings.

How are the species grouped?

The species are grouped by family and lifestyle, because fish in the same group tend to share water preferences and behaviour. Tetras are small schooling fish from soft South American water. Livebearers such as guppies, platies and mollies prefer harder, more alkaline water. Cichlids range from the peaceful discus to the large, predatory oscar. Catfish and bottom dwellers work the substrate, and shrimp and snails form the clean-up crew that many fish see as food. Each group page compares its members and lists the tank mates that suit the group as a whole.

Is a "community fish" label enough to go on?

No, a "community fish" label is not enough, because it describes temperament only and says nothing about water or size. Neon tetras and discus are both described as peaceful, yet they share no temperature range: neons are kept at 70–77 °F (21–25 °C) and discus at 79–86 °F (26–30 °C). The pair is incompatible before temperament is even considered.

Size is the other common trap. A fish in a shop tank is rarely full grown, so compatibility should be judged on adult size, which is the figure every species page and verdict on this site uses. An Oscar sold at a few inches still grows to 14 in (35.6 cm) and needs at least 75 US gal (284 L).

The opposite case also exists. Some pairs that sound risky score well: a betta with a bristlenose pleco scores 7 out of 10, because the pleco lives on the bottom, is peaceful and shares the betta's water across 72–79 °F (22–26 °C).

What can a compatibility verdict not tell you?

A compatibility verdict cannot predict how one individual fish will behave. It is built from published species-level data, so it describes the typical case: a betta that ignores shrimp, or a tetra school that never nips, are both possible outside the averages. Treat compatible as a strong starting point, caution as a pairing that needs space, cover and watching, and incompatible as one to avoid.

Verdicts also cover freshwater species only. Saltwater and reef fish, brackish species and pond stocking are outside the site's scope. Within freshwater, the verdicts assume a cycled tank, stable temperature and a stocking level the filter can handle; the aquarium stocking calculator checks that last point for a full list of fish.