The short answer
Chasing is usually about territory, breeding, or an incompatible mix of fish. A lot of it is normal and harmless — but relentless chasing that stops another fish eating or damages its fins is bullying and needs action. Start by understanding the species involved and whether the tank is set up to spread aggression out.
Normal chasing vs. bullying
Short bursts of chasing are common when fish establish a pecking order, defend a small territory, or spawn. Many active species simply dash around. It crosses into bullying when one fish is singled out constantly, kept away from food, hides all the time, or shows nipped fins and clamped-up stress. Watch for a few days to tell which you’re seeing.
Common triggers and how to calm things
- Overstocking or tight quarters raise tension — check how do I know if my aquarium is overstocked?.
- Too few of a schooling species makes aggression concentrate on one target; keep proper-sized groups.
- Territory — rearranging decor, wood and plants breaks up claimed patches and adds sightline breaks.
- Breeding makes normally peaceful fish protective of a spot; it usually passes.
- Wrong tankmates — some species are just too boisterous or aggressive for a peaceful community.
When the mix is simply wrong
If you’ve added cover, corrected group sizes and eased stocking but one fish keeps attacking, the combination may not work. Bettas in particular have strict tankmate rules — see can a betta live with other fish? and good tankmates for a betta. Persistent stress from bullying weakens fish, so also keep the water clean with regular water changes. For a tank you can’t settle, ask an experienced fishkeeping community about rehoming or re-stocking.