Fin rot: why the fins are fraying and how to stop it
Ragged, shrinking fins with a white or red edge are a bacterial infection with a water-quality cause. Here is how to tell it from nipping, and the two-part fix that actually works.
What fin rot looks like
The edge of the fin goes first. It turns milky white or, in more advanced cases, red and inflamed, and the fin itself looks frayed, as if it had been torn — then, over days, it gets shorter. The tail is usually first because it is the biggest target, but any fin can go. In a bad case the rot reaches the base of the fin and the body behind it, and at that point it is a whole-body infection rather than a fin problem.
Fin rot is bacterial — usually Aeromonas or Pseudomonas, bacteria that live in every aquarium and are harmless to a healthy fish. They get a foothold when the fish’s slime coat is damaged and its immune system is busy. Almost always that means poor water: ammonia or nitrite that has been above zero for a while, nitrate that has been drifting up for weeks, a temperature that has been too low. Fin rot is less a disease you catch than a report on the water.
Nipping is the thing to rule out. A fin that has been bitten by a tankmate is ragged but clean-edged, with no white or red border, and the damage is on one fin or one side rather than spreading evenly. A betta, a long-finned goldfish or an angelfish in a tank with barbs, tetras or a boisterous gourami has nipped fins, not rot — though nipped fins get rot next, because the wound is an open door. Watch the tank for ten minutes at feeding time and you will usually see who did it.
The fix has two halves, and the first is the water
No medication cures fin rot in bad water. Treat the fins and leave the cause, and the rot comes back the week the course ends.
Test today. Ammonia, nitrite, nitrate, pH and temperature. The numbers the app calls safe are 0 for ammonia and nitrite, nitrate under 20 ppm and certainly under 40, and for most tropical community tanks 23–27 °C.
Change water — properly. A 30–50 % change now if ammonia or nitrite is above zero or nitrate is over 40 ppm, then 25 % every two or three days for a fortnight. Match the temperature, dechlorinate, and vacuum the gravel while the water is out: rotting food and waste in the substrate is where the bacteria multiply. Clean the filter in tank water, never tap water, and never in the same week you change the substrate — the bacteria you want live in both.
Fix what pushed the water there. Overstocking, overfeeding, a filter that is too small, a tank that gets a big water change once a month instead of a modest one every week. If nitrate climbs 20 ppm between weekly tests, either the changes are too small or the tank has too many mouths in it.
In the app — log the test and the verdict tells you which readings are the problem and by how much; Do a water change opens the calculator already set to your tank’s volume and shows where the change will land your nitrate. If the readings are in the red, the schedule advisor suggests testing every two days until they clear, and the reminder arrives at the hour you chose.
The second half: the fins
Mild fin rot — a white edge, a little fraying, no redness, the fish otherwise eating and active — usually heals on clean water alone, over two to three weeks. Aquarium salt at one tablespoon per 20 litres (5 gallons) helps the slime coat recover and stresses the bacteria, and is safe for most fish; skip it for scaleless species, live plants and invertebrates. Watch the edge of the fin: regrowth shows as a clear, glassy margin, and that is the sign to stop worrying.
Moderate to severe fin rot — red, inflamed edges, fins visibly shorter week on week, rot reaching the body, a fish that has stopped eating — needs an antibacterial. A broad-spectrum aquarium antibiotic (erythromycin or a kanamycin-based product where sold, or a Maracyn-type treatment) dosed for your actual water volume, with the activated carbon removed from the filter first. Complete the full course on the label; stopping when the fins look better leaves the survivors to come back resistant. Feed lightly during treatment, and keep up the water changes — re-dose for the volume you replace.
A hospital tank makes both halves easier: a bare 20-litre tank with a heater, an air stone and a sponge filter lets you medicate one fish rather than sixty litres of community, and a daily 50 % change there is a two-minute job.
In the app — a checkup with a photo of the fin and your readings gives the severity and a treatment plan sized to it; Add plan to my schedule books the daily water changes and the medication days as tasks, and the follow-up check-in asks whether the fins are growing back.
How long it takes
Fins regrow slowly — a few millimetres a week — and the new tissue is clear before it takes colour. A tail that has lost half its length will take a month or two to look right, and a badly rotted fin may never regain its original shape, but a fish that is eating, active and no longer losing tissue has recovered. The water test a week after the fins stop receding is the one that matters: if it is still clean, the cause is gone too.