Fish Compatibility Basics: The Four Rules That Decide Everything
Apply the checks in this order
1. Water parameters must overlap
Temperature, pH and hardness. If two species have no shared comfortable window, nothing else matters — one of them will be living at the edge of its tolerance permanently. This is a slow killer: the fish does not die in a week, it dies of a secondary infection in four months and nobody connects the two.
A useful shortcut: pick species from the same broad region and water type. Soft-water South American species work together; hard-water livebearers work together; mixing them means compromising both.
2. Adult size ratio
The working rule is that anything which fits in another fish's mouth will eventually end up there. A size ratio above roughly 3:1 is a predation risk regardless of how peaceful the larger fish is rated. Angelfish and neon tetras is the classic example — sold together constantly, and the tetras disappear one by one as the angelfish matures.
3. Temperament
Peaceful, semi-aggressive and aggressive do not mix freely. Note that the risk runs both ways: fast, boisterous fish stress slow, long-finned ones even without any actual aggression, simply by outcompeting them for food and never letting them settle.
4. Swimming zone
Top, middle, bottom. Species that share a zone compete for the same space. A well-balanced community spreads across all three, which also makes the tank look fuller without adding bioload.
Three myths worth killing
“They were fine together in the shop.” Shop tanks are overstocked, which suppresses territorial behaviour, and the fish are juveniles. Both change at home.
“A bigger tank fixes aggression.” It reduces it. It does not fix a fundamental parameter mismatch or a 5:1 size difference.
“My fish is the exception.” Sometimes true, but you are betting another animal's life on it. Plan for the species, not the individual.
Check any specific pairing
We have generated a compatibility assessment for every one of the 630 possible pairings between the 36 species covered on this site. Use the compatibility checker, or browse examples such as Angelfish with Neon Tetra, or Betta with Cherry Shrimp.
Frequently asked questions
How do I know if two fish are compatible?
Check four things in order: whether their temperature, pH and hardness ranges overlap; whether the adult size ratio is under about 3:1; whether their temperaments match; and whether they occupy different swimming zones.
Can peaceful fish still be a bad match?
Yes. A peaceful fish large enough to swallow another will still eat it, and fast peaceful species routinely outcompete slow ones for food to the point of starvation.
Does a bigger tank solve incompatibility?
It reduces aggression and territorial conflict, but it does not solve a water parameter mismatch or a large size difference.
Why did my fish get along for months and then start fighting?
They matured. Most aggression and territorial behaviour appears at sexual maturity, and most shop fish are juveniles when sold.
Next steps
Written and reviewed by Marcus Rivera. See our methodology for how figures on this site are derived.