How to Choose Tank Mates That Actually Get Along
Most compatibility failures are not fights. They are slow mismatches — a temperature one species tolerates and the other is worn down by, or a fish that grew large enough to swallow its neighbour.
In This Guide
The Five Checks, in Order
Run every candidate pairing through these in sequence. If a pair fails an early check, nothing later can rescue it.
- •Water parameters — temperature and pH ranges must genuinely overlap.
- •Adult size — if one fits in the other's mouth, it eventually will be.
- •Temperament — aggression, fin nipping and speed at feeding time.
- •Swimming level — crowding is about the layer fish share, not the tank volume.
- •Group size — a schooling fish in the wrong numbers behaves like a different species.
This is the order the site uses
The tank mate suggestions on every species page are computed with exactly these rules: overlapping temperature and pH ranges, compatible temperaments, a tank the pair can realistically share, and neither species in the other's incompatible list.
1. Water Parameters Must Overlap
This is the check people skip, because two peaceful fish look like they should get along. Temperature is the usual dealbreaker.
| Species | Minimum tank | Adult size | Temperature | pH | Group size |
|---|---|---|---|---|---|
| White Cloud Mountain Minnow | 10 gal / 38 L | 1.5 inches / 3.8 cm | 60-72°F / 16-22°C | pH 6.0-8.0 | 8+ |
| German Blue Ram | 20 gal / 76 L | 3 inches / 7.6 cm | 78-85°F / 26-29°C | pH 5.5-7.0 | — |
| Peppered Corydoras | 15 gal / 57 L | 2.5 inches / 6.3 cm | 64-75°F / 18-24°C | pH 6.0-7.5 | 6+ |
| Discus | 55 gal / 208 L | 8 inches / 20 cm | 82-88°F / 28-31°C | pH 5.5-7.0 | 5+ |
A white cloud stops at 22°C and a ram starts at 26°C. There is no setting that suits both.
A useful rule: the overlap needs to be at least 2-3°C wide, not just touching at one end. Fish kept at the extreme edge of their range are permanently stressed even though nothing looks wrong.
"Tolerates" is not "thrives in"
Published ranges are survival ranges. A fish living at the last degree of its tolerance has no reserve left for disease or a heater failure.
2. Adult Size, Not Shop Size
Fish are sold as juveniles. An Oscar is sold at 5 cm and reaches 35 cm; an Angelfish is sold the size of a coin and reaches 15 cm tall, at which point neon tetras become food.
The working rule is the mouth test: if the smaller fish fits in the larger one's mouth at adult size, it will eventually be eaten — regardless of how peaceful the larger species is described as being.
- •Look up adult size before buying, not after.
- •Assume roughly a 3:1 size ratio is the maximum for peaceful species sharing a tank.
- •Remember that predation is not aggression. A peaceful angelfish eating a neon tetra is behaving normally.
3. Temperament and Fin Nipping
Three distinct problems get filed under 'aggression', and they need different solutions.
Territorial aggression
Cichlids defending a breeding site. Fixed by footprint, line of sight breaks and hiding places.
Fin nipping
Barbs and some tetras nipping trailing fins. Fixed by larger groups — a school of ten nips each other instead.
Feeding competition
Fast fish stripping the tank before slow ones eat. Fixed by feeding in two places and using sinking food.
Long-finned fish are the usual victims. A Betta Fish, Guppy or Sailfin Molly should not share a tank with Tiger Barb or Serpae Tetra, both of which are persistent nippers.
4. Swimming Levels and Territory
Fish do not use a tank evenly. Stocking across three layers rather than one is the difference between a tank that looks full and one that feels crowded to its occupants.
| Level | Typical species | What they need |
|---|---|---|
| Top | Gouramis, hatchetfish, killifish | Surface access, floating plants, a lid |
| Middle | Tetras, rasboras, danios, barbs | Open swimming length, especially for danios |
| Bottom | Corydoras, loaches, plecos | Soft sand or smooth gravel, and floor area |
Substrate matters for bottom dwellers
Corydoras and kuhli loaches sift substrate with delicate barbels. Sharp gravel wears the barbels away and leads to infection — sand or smooth, rounded gravel only.
5. Group Sizes
A schooling fish kept alone or in pairs is not a smaller version of a school — it is a stressed animal. It hides, loses colour, eats poorly, and in nippy species it redirects that behaviour onto its tank mates.
- •Six is the practical minimum for most schooling species; ten is noticeably better.
- •Tiny species such as Chili Rasbora want ten or more before they behave naturally.
- •Nipping species such as Tiger Barb become far less trouble in groups of eight or more, because the behaviour stays within the group.
- •Plan stocking in whole groups. Six corydoras is one stocking decision, not six.
Combinations That Reliably Fail
| Combination | Why it fails |
|---|---|
| Betta + guppies | The betta reads bright, long-finned males as rival bettas. |
| Tiger barbs + any long-finned fish | Persistent fin nipping, even in adequate groups. |
| Angelfish + neon tetras | Fine for months, then the angelfish is large enough to eat them. |
| Goldfish + tropical fish | Goldfish want 18-22°C; tropical species want 24-27°C. |
| Shrimp + most cichlids | Shrimp are food. Only the smallest, most peaceful dwarf cichlids are an exception. |
| African rift-lake cichlids + South American cichlids | Incompatible hardness and pH, and very different aggression styles. |
| Two male bettas | They will fight until one dies. This is not avoidable with tank size or decor. |
Every species page on this site lists both its computed tank mates and the groups to avoid. For a specific combination, the compatibility checker will test a whole stocking list at once.
Frequently Asked Questions
How do I know if two fish are compatible?
Check five things in order: their temperature and pH ranges must overlap by at least 2-3°C, neither should fit in the other's mouth at adult size, their temperaments must match, they should prefer different swimming levels, and each must be kept in its own required group size.
Can a betta live with other fish?
Often, in a tank of 20 gallons or more with small, peaceful, short-finned species such as rasboras and corydoras. Avoid brightly coloured or long-finned fish such as guppies, which a betta may attack as rivals, and never keep two males together.
Why is my peaceful fish eating its tank mates?
Because predation is not aggression. Any fish will eat something that fits in its mouth, so a peaceful species that grows large — an angelfish, for instance — will eventually eat neon tetras it lived alongside as a juvenile.
How many fish make a school?
Six is the practical minimum for most schooling species and ten is better. Very small species such as chili rasboras need ten or more before they school naturally, and nippy species such as tiger barbs are much better behaved in groups of eight or more.
Test your combination
Add every species you're planning and check the whole list at once.