The short answer
Fish get stressed by unstable water, no hiding places, aggressive tankmates and sudden change. Lower stress by keeping water parameters steady, adding plants and cover, choosing compatible fish, fitting a lid, and making any changes gradually. A calm fish eats well, shows full colour and swims openly — that’s your goal.
Start with the water
Poor or swinging water quality is the number one hidden stressor. Fish can’t tell you the ammonia is rising, but their bodies feel it. Aim for 0 ammonia, 0 nitrite and low, stable nitrate, with temperature and pH that don’t lurch around. The honest way to know is to test — a liquid test kit takes the guesswork out, and a steady weekly water change keeps things level. Match new water for temperature and dechlorinate it every time.
Give them cover and calm
A bare tank in a bright, busy room leaves fish nowhere to feel safe. Add plants, driftwood or caves so shy fish have somewhere to retreat — counterintuitively, fish with good cover spend more time out in the open because they feel secure. Keep the tank out of direct sunlight and away from slamming doors or a loud TV. A lid matters too: it stops jumpy fish leaping out and keeps the surface stable.
Get the tankmates right
Bullying is stressful and constant. Keep shoaling fish (tetras, corydoras, rasboras) in proper groups so aggression is spread out, and avoid mixing fin-nippers with slow, long-finned fish. If one fish is being chased relentlessly, rehoming or rearranging the decor often resets the pecking order.
Finally, go slow with change. Acclimate new arrivals properly (see how to acclimate new fish), add fish a few at a time, and avoid big same-day overhauls. If a fish already looks unwell, check our guide on how to tell if a fish is sick.