Fin Rot in Aquarium Fish: Signs, Causes and Treatment
Posted by Fish-Mox Team on
Fin rot is a bacterial infection that eats away at fin tissue, usually after something has weakened the fish first. Caught early, clean water alone often stops it. Once the edges turn red or the rot moves toward the body, it needs an antibacterial.
Last updated November 3, 2026. Written and reviewed by the Fish-Mox Team.
What fin rot looks like
It progresses in recognizable stages:
- Early. Fin edges turn milky, white, or slightly dark. The fins look a little ragged but the fish acts normal.
- Active. Edges fray and split, rays poke out past the tissue, and a red or bloody line appears where healthy fin meets damaged fin.
- Advanced. The rot reaches the base of the fins and the body. The fish clamps its fins, hides, and stops eating.
Don't confuse it with fin nipping, which leaves clean bites, or with regrowth. New fin tissue is clear and thin at the edges, and it can look alarming if you've never seen it before.
Why it starts
The bacteria behind fin rot, mostly Aeromonas, Pseudomonas, and the columnaris bacterium, live in nearly every aquarium. They take hold when a fish's defenses drop:
- Ammonia, nitrite, or high nitrate from an overloaded or new tank
- Stress from shipping, crowding, or aggressive tank mates
- Torn fins from nets, decorations, or fights
- Water that's too cold for the species
Treating with an antibacterial without fixing the cause usually means the rot comes back.
First steps, before any treatment
- Test the water. Ammonia and nitrite should be zero, nitrate low.
- Change water. A 25 to 50 percent change with temperature-matched, dechlorinated water lowers the bacterial load.
- Remove the cause. Separate fin-nippers, smooth sharp decor, reduce crowding.
- Watch for 2 to 3 days. If the edges stop receding and no red line appears, clean water may be enough.
When to use an antibacterial, and which one
Move to an antibacterial if the rot keeps advancing in clean water, if you see red streaks or a bloody edge, or if it has reached the body.
| Situation | Common choice |
|---|---|
| Early or mild fin rot, cause unclear | Fish Mox or Fish Pen |
| Red fin bases, inflamed tissue, or no response to amoxicillin | Fish Flex |
| Fast spread toward the body, bloody edges | Fish Flox or Fish Sulfa Forte |
| White, cottony edges with patches on the body or mouth | Fish Zithro or Fish Doxy |
| Keeps coming back after a full course | Fish Doxy, and talk to an aquatic vet |
Treat in a hospital tank if you can, dose by the water volume on the product label, and finish the course.
What recovery looks like
Within 3 to 5 days of starting an antibacterial, the red edge should fade and the rot should stop advancing. New growth appears as a clear or whitish band at the fin edges over the next 1 to 3 weeks. Full regrowth can take a month or more, and very damaged fins may grow back slightly uneven. Keep the water clean during regrowth; that matters more than any additional treatment.
Preventing it
- Weekly partial water changes and a filter sized for your stocking
- Quarantine new fish for 2 to 4 weeks before adding them
- Compatible tank mates, especially for long-finned fish such as bettas and fancy goldfish
- Stable temperature in the right range for the species
Sources
- Merck Veterinary Manual, Bacterial Diseases of Fish
- Declercq et al., Columnaris disease in fish: a review with emphasis on bacterium-host interactions, Veterinary Research
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