The issue of fantail fins turning black is quite common and might be cause for alarm or completely harmless.
Major changes in normal habits along with changing color are signs there could be a serious problem.
In most cases the black color is a sign the healing process has already begun, and further treatment is not necessary.
Causes of your goldfish fins turning black
#1 Ammonia Burns
This is the most common reason and is a result of the tank not being properly cleaned. Food that’s not eaten and fish waste causes toxins to build in the water.
If the water isn’t properly filtered and cleaned often, the toxins can cause burns to the goldfish. The black color is a sign the infection is healing, and cleaning out the tank properly will help ensure the infection doesn’t return. In time, the goldfish should return to normal color.
#2 Goldfish can naturally change color
It’s possible that there’s nothing wrong at all. Goldfish are known to change colors. The change could be just a normal process for the fish.
#3 The tank or aquarium is too small
The tank isn’t large enough for the number of goldfish. People often begin with a small tank or bowl when buying their first fish. The water very quickly gets stagnant if it’s not cleaned regularly. Toxins then infect the tank and cause illness.
#4 Water temperature
It’s hard to keep the water temperature regulated in a small bowl. The small size of the tank easily warms and causes infections in goldfish and you’ll find your fantails fin turning black.
Simply investing in a larger tank minimizes this problem.
Purchasing a 10 or 20 gallon tank for your fish is generally sufficient to ensure the goldfish has plenty of room to swim. The size recommended depends of course on the number of fish living in the tank.
It’s also easier to install a filter system to a larger tank.
If the fish is acting normally, the water is clean, and the water temperature is fine, there’s probably no reason for concern.
If you’re still worried it’s best to completely clean out the tank because keeping a clean fish tank is the ideal way to avoid diseases and unnecessary infections.