Ulcers and Red Sores on Fish: Causes and Treatment
Posted by Fish-Mox Team on
Red sores and open ulcers on a fish are almost always bacterial, and they can go from a pink spot to a deep crater in a few days. Fast, gram-negative bacteria are the usual culprits, which shapes which antibacterial to choose.
Last updated November 3, 2026. Written and reviewed by the Fish-Mox Team.
How ulcers develop
- A red spot or patch appears, often where scales are missing or the skin was scraped.
- The scales lift and fall away and the spot turns into a shallow open sore.
- The sore deepens into a crater with a white or red rim, sometimes exposing muscle.
Some ulcers stay single and slow. Others spread to several fish in a week. How fast it moves is your best clue to what you're dealing with.
Common causes
- Aeromonas and Pseudomonas in freshwater, and Vibrio in marine tanks. The Merck Veterinary Manual lists these as the bacteria most often isolated from sick aquarium fish.
- Injuries from nets, decor, or fights, which give bacteria a way in
- Parasites such as anchor worms or fish lice, which leave wounds behind
- Chronic stress from poor water, crowding, or wrong temperature
Matching the treatment to the sore
| What you see | Common choice |
|---|---|
| One shallow sore after an injury, healing slowly | Fish Flex or Fish Pen |
| Red ulcers that deepen or spread from day to day | Fish Flox |
| Ulcers on several fish in a large tank | Fish Sulfa Forte (one tablet covers 40 gallons) |
| A swollen, pus-filled lump or a deep puncture wound | Fish Cin |
| Sores with no clear pattern, mild signs | Fish Mox |
If you see a parasite attached to the sore, remove or treat the parasite too; otherwise the wound keeps reopening.
Treatment routine
- Isolate affected fish in a hospital tank. Ulcer bacteria spread through shared water.
- Remove carbon and UV from the treatment tank's filter.
- Dose by water volume following the product label, once a day with a partial water change before each dose.
- Check on day 5. The red rim should fade and the sore should stop growing.
- Finish the course and keep the water clean while the skin closes over.
How long healing takes
Shallow sores close over in 1 to 2 weeks once the infection is controlled. Deep ulcers take longer and may leave a scar or a patch of missing scales. A gray or white film over a healing ulcer is usually new skin, not a fungus, as long as it isn't fluffy.
Keeping it from coming back
- Find and fix the stress that let the bacteria in: water quality, crowding, aggression
- Quarantine new fish, since many ulcer outbreaks start with a new arrival
- Disinfect nets and buckets between tanks
Sources
- Merck Veterinary Manual, Bacterial Diseases of Fish
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