What Does It Mean When a Fish's Fin Has a Rotted Hole in It?

A rotted hole in the fin of a fish demands quick action. Rotting damage to fins is fin rot, a serious infection that can rapidly eat a fin down to a ragged stump and proceed to damage the fish's body. Death can occur within a week of an infection's start. You need to start treating the fish as soon as you see this symptom.

  1. Fin Rot

    • Fin rot is a nonspecific opportunistic infection. This means a number of normally harmless organisms can cause this infection, including bacteria and fungi found in most aquarium water. These organisms cannot infect a healthy fish, causing problems only to a fish weakened from stress or injury. These organisms invade open wounds or damaged tissue, then begin to grow and spread. As they grow, they produce chemicals that damage surrounding tissue, allowing them to spread deeper into the fish.

    Treatment

    • Most pet shops sell antibacterial and antifungal drugs intended to treat fin rot. However, these products can damage the biological filter -- the beneficial bacteria living in the tank that eat fish waste -- so you may want to treat the infected fish in a separate aquarium. Fortunately, fin rot does not readily spread between fish, so you do not have to worry about treating the entire aquarium. This makes the idea of treating the fish in a separate aquarium even more appealing.

    Causes

    • Fin rot can occur only if certain problems are already present. For example, poor water conditions can foster the growth of fin rot. High levels of ammonia and other toxic nitrogen products can promote damage to the skin and fins of fish, creating a breeding ground for secondary infections. You also need to watch fish for fin rot when they've recently been handled, such as new fish that have just come from the fish store. Additionally, watch for aggression between fish, since fight injuries often lead to fin rot.

    Prevention

    • You can take several proactive steps to avoid fin rot. First, keep up on aquarium maintenance. This keeps levels of fish waste down, which can help keep fish healthy, unstressed and fin rot-free. Observe your fish closely after transferring them or handling them in a net. Also, observe your fish closely to look for aggression. You may also be able to prevent fin rot by adding 1 teaspoon of salt per gallon of water as a tonic. Keep in mind that some species of fish respond poorly to this; always research your specific fish before doing this.