Samuel Chiltern answered
There are a few possible explanations for black lips on a goldfish. It may be that this is natural pigmentation which is emerging as your fish matures, but it could also be indicative of one of the more serious problems explained below.
Injury
Black lips might be caused by an injury, in much the same way as humans bruise when they suffer physical trauma to surface tissue.
Water Quality
The quality of the water in your tank could be affecting your fish. If the fish is displaying erratic or sluggish behavior, then I'd check to make sure that the water is being properly cycled and filtered.
If you don't have a water filter, then you should buy one which is large enough to fully cycle the water in your tank.
Infection and Ammonia Burn
Ammonia burn is a level of toxicity caused by high levels of ammonia in the water. This problem can be caused by inadequate filtration, giving too much food or even using an ammonia-based cleaning product in the vicinity of the tank.
The ammonia actually burns the surface areas surrounding the eyes and mouth of the fish, and leaves these areas open to bacterial infection.
If your fish is displaying other characteristics such as frayed fins, cloudy eyes or a lack of appetite then this ammonia burn is a likely explanation.
The best treatment in this case is to isolate the fish (you might only have one anyway) and then treat with a suitable antibacterial medication.
Don't be tempted to use antibiotics, since these will kill all the bacteria in the tank, including the beneficial and innocuous bacteria which make your fish's environment healthier.
The best way to prevent this from happening in the future is to use some form of ammonia buffer to keep the ammonia levels down.
Injury
Black lips might be caused by an injury, in much the same way as humans bruise when they suffer physical trauma to surface tissue.
Water Quality
The quality of the water in your tank could be affecting your fish. If the fish is displaying erratic or sluggish behavior, then I'd check to make sure that the water is being properly cycled and filtered.
If you don't have a water filter, then you should buy one which is large enough to fully cycle the water in your tank.
Infection and Ammonia Burn
Ammonia burn is a level of toxicity caused by high levels of ammonia in the water. This problem can be caused by inadequate filtration, giving too much food or even using an ammonia-based cleaning product in the vicinity of the tank.
The ammonia actually burns the surface areas surrounding the eyes and mouth of the fish, and leaves these areas open to bacterial infection.
If your fish is displaying other characteristics such as frayed fins, cloudy eyes or a lack of appetite then this ammonia burn is a likely explanation.
The best treatment in this case is to isolate the fish (you might only have one anyway) and then treat with a suitable antibacterial medication.
Don't be tempted to use antibiotics, since these will kill all the bacteria in the tank, including the beneficial and innocuous bacteria which make your fish's environment healthier.
The best way to prevent this from happening in the future is to use some form of ammonia buffer to keep the ammonia levels down.
