Black patches can be ammonia burns that are healing, parasites, bacterial, injury.
Some information on black patches.
Black Patches
Symptoms:
Black patches, smudges, smears, spots or markings that suddenly appear and are not natural colorings.
Cause:
These markings are the result of skin cell damage. Black markings often appear in areas where the skin has been damaged by ammonia burns, bacterial infections, parasitic infestations, and injuries. These markings are often seen on fish just after a new tank has completed �cycling,� due to the high ammonia levels. These marks are also common in African Cichlids, especially around the mouth. These are scabs that have come from digging in the gravel or from fighting.
Treatment:
No treatment is necessary. Much like scabs, the blackened areas are a sign that the healing process has already begun. Keep water in top condition and the areas will gradually fade in less than a couple of weeks.
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