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Worst case: Act within days5 possible causes

One fish is attacking the others

A fish chases, corners or damages tankmates.

Short answer

Most aquarium aggression is a stocking problem rather than an individual's temperament. Group size, tank dimensions, sex ratio and breeding state explain the large majority of cases.

Check these first, in this order

  1. Which fish, towards which? Same species or different?
  2. Are the group sizes correct for the species involved?
  3. Is the tank long enough for separate territories?
  4. Has anything changed — new fish, rearranged decor, a pair forming?

Possible causes

Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.

Shoaling species in too small a group

Act within daysvery common

How to tell: Tiger barbs, serpae tetras, black skirt tetras or similar in groups of three to six. Aggression that would be absorbed within a large shoal is redirected outwards.

Check

  • Group size against the species minimum

Response

  • Increase the group to the species minimum or beyond
  • Expect a change within days

    Breeding aggression

    Act within daysvery common

    How to tell: A pair has formed and is defending an area. Common in cichlids and it appears suddenly in a previously peaceful tank.

    Check

    • Whether two fish are pairing or guarding a site
    • Whether the aggression radiates from one spot

    Response

    • Remove either the pair or the harassed fish.
    • Break sightlines with hardscape.
    • This is temporary in some species and permanent in others.

      Insufficient space or territory

      Act within daysvery common

      How to tell: Territorial species in a tank too short to contain more than one territory.

      Check

      • Tank length against the species' requirement
      • Whether there are visual barriers

      Response

      • Rescape to break the tank into visually separate areas
      • Rehome to a larger tank
      • Reduce the number of territorial individuals

      Sex ratio

      Act within dayscommon

      How to tell: Male livebearers pursuing a single female, or several males of a harem species with too few females. The target is consistently the same individual.

      Check

      • Sex ratio
      • Which individual is targeted

      Response

      • Adjust the ratio — more females, or a single-sex group
      • Rehome surplus males

        Genuinely incompatible species

        Act within dayscommon

        How to tell: The species combination was never workable — a predator with prey-sized fish, a strongly territorial fish with peaceful ones.

        Check

        • Check the pairing against a compatibility assessment

        Response

        • Separate them. No amount of rearrangement fixes an incompatible pairing.

        What not to do

        • Adding more fish to 'spread the aggression' in a tank that is already too small
        • Assuming a fish will grow out of it

        Prevention

        • Check compatibility before buying
        • Buy proper group sizes
        • Plan for adult size and for breeding behaviour