Likely cause

Aggression / Stocking Issue

Ongoing aggression toward tankmates is usually a stocking problem — not enough space, incompatible species, or not enough hiding spots and broken sightlines for weaker fish to retreat to.

Why this happens

Not enough tank volume for the number or size of fish present.
Incompatible species mixed together — different temperaments, activity levels, or territorial needs.
Too few hiding spots or open sightlines, which stop weaker fish from breaking line of sight with an aggressor.
An imbalanced male-to-female ratio in species where males compete over females.

What to do next

Research the adult size and temperament of every species in the tank against your actual tank size.
Add more decor, plants, or caves to break up sightlines and create hiding spots.
Consider rehoming or separating a persistently aggressive individual if the behavior doesn't settle.
Rearrange decor when reintroducing a separated fish — this resets territory boundaries.

Frequently asked questions

How do I stop fish from fighting?

Add more hiding spots and visual breaks first — most aggression is territorial and eases once fish can't constantly see each other.

Is some aggression normal in a community tank?

Brief chasing or fin-flaring during initial introductions is common; ongoing chasing, biting, or a fish hiding constantly is not and needs addressing.