Guppies are generally social and peaceful, which is why aggression surprises people. If guppies start chasing constantly, nipping fins, or harassing a weaker fish, it usually isn’t because they “became mean”. It’s because something in the environment rewards that behaviour. Fix the trigger and the tank often settles quickly.
1) Too-small group (pairs are risky)
Two males often develop a constant rivalry. Small groups can also create a single target. A slightly larger group spreads attention and reduces focused bullying.
2) Overcrowding
More fish means more competition. Even if water tests look okay, constant contact increases stress and chasing behaviour.
3) Too much open space
Open tanks allow straight-line pursuit. Add plants or decor to break line-of-sight and create “escape lanes”.
4) Feeding competition
When food hits one spot, the bold fish dominate and the shy fish lose out. Scatter-feeding reduces dominance.
5) Wrong tank mates
Some tank mates trigger guppies into defensive behaviour, and some tank mates are the real nippers while guppies get blamed. Watch closely during calm periods, not just at feeding time.
6) Strong flow
Long tails struggle in strong current. A tired fish is an easy target. Gentle, even flow reduces stress and chasing.
7) Chronic stress from instability
Unstable temperature, low oxygen, or fluctuating pH makes fish irritable. When fish are comfortable, behaviour improves.
Aggression in guppy tanks is usually a design problem, not a personality problem. Adjust space, cover, feeding, and flow — and you’ll often see the tank become calm within days.
