Aquarium fish compatibility is about more than temperament. Two fish genuinely do not belong together when their water needs do not overlap, when one grows big enough to eat or bully the other, when one nips fins and the other has long ones, when a territorial species has no room to claim, or when males of the same species fight. Most store charts only check temperament, labeling fish "peaceful" or "semi-aggressive". That is why a pair the chart called compatible can still fail within weeks.
My name is Hossain. On this site, I explain why before I tell you what, so each mismatch below comes with the reason it causes trouble and, where a number is involved, the source it came from.
Why the Chart Said Compatible
The reader question behind this article is usually "The chart at the shop said compatible. It clearly was not." Charts are not lying. They are answering a narrower question than the one you asked.
A typical compatibility chart sorts fish by temperament and then says which temperament groups can share a tank. What it usually does not check:
- Whether the two species come from similar water. A peaceful fish kept too warm or too cold is a stressed fish.
- How big each species gets as an adult. Fish are sold young and small.
- Fin shape. A peaceful fin nipper is still a fin nipper.
- Territory and same-species aggression. Some fish are calm with other species and fight their own kind.
Treat a chart as a first filter. Then check the five mismatches below yourself.
For the figures in this article I used FishBase, a scientific database that records maximum length, temperature and water ranges, and aquarium notes for each species. I read every species entry mentioned here on September 27, 2026. Those ranges describe the species and its native waters. They are not "safe" levels, and fish raised on farms may be used to different conditions. They are still the best tool I know of for spotting a mismatch before it happens.
Mismatch 1: Water Temperatures That Do Not Overlap
This is the most overlooked problem in aquarium fish compatibility, because "tropical" sounds like one temperature. It is not.
FishBase records a temperature range of 18 to 23 °C (about 64 to 73 °F) for the peppered cory, and 24 to 30 °C (about 75 to 86 °F) for the betta, also sold as the Siamese fighting fish. Those two ranges do not overlap at all. Both are peaceful toward each other on most charts, and both are sold to beginners for small community tanks. Keeping them together means one of them always lives outside its recorded range.
The fix is simple: look up every species' range and make sure there is real overlap, not just a shared "tropical" label. I explain the reasoning in more detail in why "tropical" is not one number.
Mismatch 2: Water Chemistry Pulling in Opposite Directions
Temperature is not the only range that can fail to overlap. FishBase records very different home waters for two of the most popular beginner fish:
- Neon tetra: pH 5.0 to 7.0, hardness 1 to 2 dH, from blackwater and clearwater streams in South America.
- Guppy: pH 7.0 to 8.0, hardness 9 to 19 dH, from northern South America and nearby islands.
The pH ranges meet only at 7.0, and the hardness ranges do not meet at all. That does not mean the two can never share a tank, since fish raised on farms may tolerate more than their wild relatives. What the numbers tell you is that one of them will always be further from its home water than the other, and that a tank with very soft or very hard water will favor one species. My guide to GH and KH and whether you need to test them explains how to find out which way your water leans. Stability matters more than chasing either fish's numbers.
Mismatch 3: Adult Size
Fish are sold small. The question is how big they get.
FishBase records a maximum length of 2.5 cm SL (standard length, excluding the tail) for the neon tetra. For the freshwater angelfish, it records a maximum of about 10.5 cm TL (total length), with one reference giving 15 cm TL. For the goldfish, it records a common length of 10 cm TL and a maximum of 48 cm TL.
A general principle in fishkeeping is that a fish small enough to fit in a tank mate's mouth is at risk from it, whatever the chart says about temperament. A juvenile angelfish and a group of neons can look like a fine match in the store. The angelfish will not stay juvenile.
Goldfish are a special case. They are usually kept as cool-water fish, and FishBase's length figures show how large they can grow. Mixing them with small tropical fish usually combines a size mismatch with a temperature compromise that suits neither.
Mismatch 4: Fin Nippers and Long Fins
Some fish that are peaceful in the sense of not attacking will still nip at trailing fins. The clearest example in FishBase's notes is the tiger barb, listed in the FishBase entry for Puntigrus tetrazona as the Sumatra barb, which says it is "not to be kept with long-finned fishes."
Long-finned fish in beginner tanks include bettas, fancy guppies with flowing tails, and angelfish. A tiger barb group in the same tank as any of these is one of the most reliable ways to end up with ragged fins. If you already have damaged fins, it is worth working out whether the cause is nipping or disease before you act, because the responses are different. I explain how in fin rot vs fin nipping.
Mismatch 5: Territory and Same-Species Fighting
Some fish get along with other species and not with their own kind, or become aggressive once they claim a space.
- Male bettas: FishBase notes plainly that males will fight each other. Two male bettas do not belong in the same tank, however large.
- Cichlids: the University of Florida's extension circular Freshwater Ornamental Fish Commonly Cultured in Florida (Hill and Yanong, UF/IFAS, read September 27, 2026) notes that all cichlids provide some form of parental care and that many species are highly territorial. A cichlid defending a territory, or eggs, can turn on fish it ignored the week before.
- Angelfish pairs: FishBase describes pair bonding in the freshwater angelfish, with each partner defended against aggression or rivals. A pair that forms in a community tank changes the behavior of the whole tank.
Pairs That Genuinely Do Not Belong Together
Here are the combinations above in one place. Each reason comes from the FishBase and UF/IFAS entries cited in this article.
| Pair | The mismatch | Why it fails |
|---|---|---|
| Peppered cory and betta | Temperature | Recorded ranges of 18 to 23 °C and 24 to 30 °C do not overlap |
| Tiger barb and betta, fancy guppy or angelfish | Fins | FishBase says tiger barbs are not to be kept with long-finned fishes |
| Two male bettas | Same-species fighting | FishBase notes males fight each other |
| Adult angelfish and neon tetras | Size | Angelfish reach 10.5 to 15 cm TL; neons reach 2.5 cm SL |
| Goldfish and small tropical fish | Size and temperature | Goldfish can reach 48 cm TL and are kept cooler than tropical fish |
| Neon tetra and guppy in extreme water | Chemistry | Recorded pH and hardness ranges barely meet, so very soft or very hard water favors one |
Aquarium Fish Compatibility: How to Check Any Pair Yourself
Before you buy, look up each species and answer five questions:
- Do the temperature ranges overlap by a real margin?
- Do the pH and hardness ranges point the same way, and which way does your water lean?
- How big does each get as an adult, and could one fit in the other's mouth?
- Does either nip fins, and does the other have long fins?
- Is either territorial, or aggressive toward its own kind?
If any answer is a clear mismatch, the pair does not belong together, whatever a chart says. If you are unsure, wait. The fish will still be in the store next week.
Adding New Fish Safely
Even compatible fish can bring problems with them. A separate quarantine tank lets you watch new arrivals for a few weeks before they meet your existing fish, and it gives you somewhere to move a fish if temperaments turn out worse than expected. I explain why in quarantine, the cheapest insurance in the hobby.
If fighting has already started, separate the fish first, then decide on a permanent arrangement. If a fish is injured, or several are showing damage, contact an aquatic veterinarian rather than treating from a guess. The right response depends on what caused the damage.
Frequently Asked Questions
What makes aquarium fish compatible?
Similar water needs, similar adult size, compatible fins and temperament, and no territorial or same-species aggression that the tank cannot absorb. Temperament alone, which most charts check, is only one of these.
Can a betta live with other fish?
A single betta can share a tank with some species, but its tank mates need a temperature range that overlaps the betta's recorded 24 to 30 °C and must not be fin nippers. Two male bettas should never be kept together, since FishBase notes the males fight.
Can goldfish live with tropical fish?
Usually not well. Goldfish are usually kept cooler than tropical fish, and FishBase records them reaching up to 48 cm TL, so mixing them with small tropical fish combines a size mismatch with a temperature compromise.
Why are my fish fighting when the chart said compatible?
Charts mostly check temperament. Fighting often comes from territory, a pair forming and defending space, same-species aggression, or stress from water that suits one species better than another. Separate the fish and check the five mismatches in this article.
Are tiger barbs aggressive?
FishBase's aquarium notes say tiger barbs should not be kept with long-finned fishes, which is the problem most people run into. The same notes recommend keeping them in groups of 5 or more.