Fin rot is a bacterial infection that causes the edges of fins to become ragged, discolored, or progressively shorter. It is one of the most common diseases in aquarium fish and is almost always triggered by poor water quality or stress that weakens the immune system.

The first and most important step is improving water quality. Test ammonia, nitrite, nitrate, and pH immediately. Perform a 30 to 50 percent water change with dechlorinated, temperature-matched water, and continue with daily or every-other-day partial changes until parameters stabilize. In mild cases, clean water alone is enough for fins to begin regrowing within a week or two.

Remove any sources of ongoing stress such as aggressive tankmates, sharp decorations that could tear fins, or overcrowding. Ensure the temperature is stable and appropriate for the species.

For moderate cases where the rot has progressed beyond fraying edges, aquarium salt can help. Dissolve one tablespoon of aquarium salt per five gallons of water. Salt should not be used with scaleless fish, most catfish, or invertebrates, so verify species compatibility first.

Severe fin rot that reaches the base of the fin or involves redness, swelling, or fuzzy growth may require antibacterial medication. Products containing erythromycin or kanamycin are commonly used for gram-negative bacterial infections. Follow the medication instructions exactly, remove activated carbon from the filter during treatment, and maintain good aeration since some medications reduce dissolved oxygen.

After treatment, continue pristine water conditions to support regrowth. Fins can regenerate fully in healthy fish, though the new tissue may appear thinner or slightly different in color initially. Prevention through consistent water maintenance is always preferable to treatment.