Goldfish under a lot of stress might develop fin rot, a bacterial infection also known as tail rot. Fin rot is usually a secondary infection that may infect your fish if they’re already stressed from another goldfish disease or injury.
Causes of Fin Rot – Stress will make your goldfish susceptible to certain types of bacteria. Common goldfish diseases, poor water quality, overcrowding, sudden temperature changes, fin nipping, or aggressive fish can all cause the immune system to lower and fin rot to develop.
Fin Rot Symptoms – Your goldfish will show signs of fin rot if they have ragged, torn, or red-streaked fins. If allowed to get worse, the edges of torn fins will become white as bacteria eat away the fins on your goldfish. The bacteria infection might become so bad that it will completely deteriorate fins to the body tissue. If the disease is allowed to spread to the fin base, your goldfish cannot regrow their fins. So treat fin rot early!
Fin Rot Treatment – Good water quality is a must! So test that water. Your goldfish cannot start the healing process if water quality is poor. After the water environment is healthy again, treat the water with 1 teaspoon of aquarium salt per gallon of water. If you don’t notice an improvement after 5 days, move on to commercial treatments. Before treating, take out the active carbon from the filter. I recommend Maracyn-Two. It works great for many bacterial infections and uses minocycline as an active ingredient. Once you’re done with treatment, change out 25% of the water to get rid of excess salt. Continue your weekly water changes as usual.
Treat fin rot early. Don’t allow the bacteria to completely deteriorate the fins or your goldfish will not be able to grow them back. If treated quickly, you should notice signs of healing after a couple weeks.