Red streaks, sores, or ulcers
This looks like: aggressive bacterial infection — aeromonas-type pathogens love dirty margins
Why this happens
Ulcers start where mucus weakens — injury, parasites, or ammonia burn opens doors — bacteria flood tissue fast in warm water.
What to do right now
- Test ammonia nitrite immediately — zero tolerance for excuses.
- Large safe partial changes until pristine.
- Hospital tank if only one animal — treat water volume exactly.
- Antibacterial per species tolerance — follow label grams per volume.
- Improve food quality during recovery — easy protein, not giant feasts.
What not to do
- Don’t ignore because fish “still eats.”
Stop it happening again
Keep ammonia and nitrite at zero, avoid injuries and aggression, and quarantine new fish - aggressive bacteria exploit dirty water and damaged skin.
If that doesn’t fix it
Healing pink skin without fuzz is winning — white fluffy regrowth says secondary fungus.
Catch this earlier next time — log your water parameters in App-aquatic. Free forever.
This isn't veterinary advice - it's a directional read to help you decide your next step, based on the most common causes. We can't see your fish, so pair it with a liquid test kit and clear photos, and see an aquatic vet or a trusted local fish shop for a valuable or seriously ill fish.