Are These Fish Compatible? How to Check in a Minute

Two fish get along when their numbers overlap: temperature, pH, hardness, and adult size, before you ever reach temperament. A neon tetra at pH 5.5 to 7.0 and a guppy at 7.0 to 8.0 barely share a point on the scale, so one of them is always in the wrong water. Checking that overlap takes about a minute on the species cards, and it saves the fish you would otherwise lose slowly over three weeks.

Compatibility is not a vibe or a stocking chart you memorize. It is four ranges lined up side by side, plus two judgment calls about temperament and group size. Do the numbers first, because a fish in the wrong water fails no matter how peaceful it is.

The short version

• Line up four numbers for every fish: temperature, pH, hardness, and adult size. All four must overlap.
• Then check two behaviors: temperament (nippers, predators, territory) and group needs (schoolers need six or more).
• No overlap on water means no tank. A soft-water neon (pH 5.5 to 7.0) and a hard-water guppy (7.0 to 8.0) do not mix.
• Size is predation. A mouth twice the width of a tank mate eventually uses it.
• The planner does this check for you: enter the species and it flags the conflicts.

The four numbers that decide it

Pull up each fish and read four fields off the card. Temperature range, pH range, hardness range, and adult size. If any one of those four does not overlap across all your fish, the group does not work, and no amount of plants or filtration fixes it.

Temperature is first because it is the easiest to get wrong. A betta (Betta splendens) wants 78 to 82 F and a white cloud minnow wants 60 to 72 F: those ranges never touch, so the two share no viable tank temperature. Two tropical community fish usually overlap here, but a warm-water and a cool-water fish almost never do.

pH and hardness come next, and they move together. A soft-water fish reads a low range on both; a hard-water fish reads high on both. Adult size is the fourth: a fish grows into its listed size, and a big enough mouth turns a tank mate into food regardless of temperament.

A worked example: betta, neon, cory, guppy

Here are four common beginner fish, every number copied from the database, lined up the way the check wants them.

Species	Temp	pH	Hardness (dGH)	Adult size	Temperament	Min group
Betta (Betta splendens)	78 to 82 F	6.5 to 7.5	3 to 12	2.5 in	territorial	1
Neon tetra (Paracheirodon innesi)	70 to 81 F	5.5 to 7.0	1 to 8	1.2 in	peaceful	6
Bronze corydoras (Corydoras aeneus)	72 to 79 F	6.0 to 7.5	2 to 12	2.5 in	peaceful	6
Guppy (Poecilia reticulata)	72 to 82 F	7.0 to 8.0	8 to 20	2 in	peaceful	5

Run the check. On temperature, the betta (78 to 82) and neon (70 to 81) overlap at 78 to 81, and the cory (72 to 79) squeezes in; the group shares a workable band. On pH, all four overlap loosely around 7.0. On hardness, the problem appears: the neon caps at 8 dGH and the guppy starts at 8, so they meet at a single edge value, not a range. The neon and guppy want opposite water.

Temperament finishes the job. A guppy's flowing tail is a target for a betta, which the database flags directly under the betta's avoid list. So this foursome is not one tank: it is a soft-water group (neon plus cory) and a hard-water fish (guppy) that belongs elsewhere, and a betta that is better kept alone or with a cleanup crew.

Temperament and group size, the two judgment calls

Numbers overlapping is necessary but not sufficient. Two fish can share every parameter and still fail on behavior. A bronze corydoras (Corydoras aeneus) is peaceful and fits most communities, but it needs a group of six or more or it stresses and hides, and it wants a 20-gallon footprint of smooth sand. Stock one cory and you have followed the numbers and still kept the fish badly.

Temperament splits into three failure modes. Fin-nippers, like tiger barbs and some tetras, shred slow, long-finned fish. Predators eat anything that fits in the mouth, which is the size rule again. Territorial fish, like a male betta, defend space and cannot share it with a lookalike or a rival.

Group size is the quiet one. A schooling fish kept in twos and threes is not a calmer version of a school; it is a stressed fish that hides and fades. A neon tetra (Paracheirodon innesi) needs six or more, a harlequin rasbora (Trigonostigma heteromorpha) the same, and a honey gourami (Trichogaster chuna) is content as one. Read the minimum group on the card and honor it.

The honest part: where compatibility checks go wrong

The chart on the shop wall is where it usually starts. Those grids collapse every fish into "peaceful" or "aggressive" and skip water chemistry entirely, so they green-light a neon (Paracheirodon innesi) and a guppy that will never share water. The database ranges exist because "peaceful" is only half the question.

The second trap is the individual fish. A range and a temperament are averages, and animals vary. A particular betta may ignore a shrimp for a year, or hunt it the day you add it; a normally peaceful fish may turn territorial while spawning. Plan for the temperament on the card, give hiding cover, and watch the tank for the first week rather than trusting the profile blindly.

The third is buying before checking. Once a fish is in the bag it is hard to walk back, so the check belongs in the shop or, better, at home the night before. A minute with the cards beats a slow loss over the three weeks a mismatched fish takes to decline.

Frequently asked questions

What is the fastest way to check if two fish are compatible?

Line up four numbers from each species card: temperature, pH, hardness, and adult size. If all four ranges overlap, then check temperament and minimum group size. The build planner runs this automatically, flagging any conflict when you enter the species.

Do compatible parameters guarantee two fish will get along?

No. Overlapping water is necessary but not the whole story. A fish that shares every parameter can still nip fins, eat a smaller tank mate, or defend territory. Check temperament and group needs after the numbers line up, and give cover so animals can avoid each other.

Why do soft-water and hard-water fish not mix?

Because pH and hardness travel together, and the two groups want opposite water. A neon tetra sits at pH 5.5 to 7.0 and GH 1 to 8, while a guppy sits at pH 7.0 to 8.0 and GH 8 to 20. Housing them together leaves one fish outside its range long term, where it weakens.

Can a betta live in a community tank?

Sometimes, with care, and never with fin targets. A male betta (Betta splendens) is territorial and reads flowing tails, like a guppy's, as a rival, so those pairings fail. Peaceful, short-finned tank mates and a nerite snail cleanup crew are a safer starting point, and every betta is an individual you watch closely.

The check is faster than the regret, so do it before you buy. Enter your shortlist in the build planner to see the conflicts flagged, read each species range in the livestock database, part of the wider compatibility database, and if the process is new, start with how to use the compatibility database and how the build planner works.