Quick answer
Chasing can be courtship, hierarchy or territorial aggression. Identify the species and sex, check group size, space, hiding structure and injuries, then intervene before one fish is excluded from food or rest.
What to check first
- Look for torn fins and cornering
- Check whether aggression follows spawning
- Review male-to-female ratio
- Confirm the tank provides separate territories
Write down the test results, recent maintenance, new livestock, feeding changes and equipment faults. A short timeline prevents repeated guesses and makes patterns easier to see.
A practical action plan
- 1. Rearrange décor when territories are establishedRecord what changes and retest before making another major adjustment.
- 2. Add visual barriersRecord what changes and retest before making another major adjustment.
- 3. Correct group size only if the tank can support itRecord what changes and retest before making another major adjustment.
- 4. Separate incompatible individualsRecord what changes and retest before making another major adjustment.
What to avoid
Do not add more fish blindly or use a tiny breeder box as long-term housing.
When it is urgent
Continuous attacks, severe injury or inability to feed requires immediate separation.
How to prevent it next time
Use stable routines: quarantine additions, test before and after major changes, maintain backup aeration, keep equipment accessible, and record enough detail to compare today with the aquarium’s normal baseline. Prevention is usually cheaper and safer than emergency correction.
Research and review
This is original AquaNexus writing prepared for practical aquarium use. The husbandry framework was checked against authoritative veterinary, safety, scientific or specialist references.
Test rather than guess and make changes at a pace the livestock can tolerate. This guide does not replace aquatic veterinary advice. Follow product labels and local electrical-safety requirements.