Why are the gills on my oscar turned inwards?

Question

we have a beautiful tiger oscar. hes about 1 year old and about 8 inches long. he is in a 37 gallon tank with a small gourami and and small golden nugget pleco. i noticed recentally that the gills on his right side are not formed right. the smaller gill slit seems to be completely sealed shut and the outer gill is curling inwards like a sheet curled under the mattress. making the gill opening stick out from the body. can anyone please tell my why this is happening and if its dangerous? he seems fine other than that. he eats and does things as though nothing is wrong with him.

Answer 1

Gill Curl, more often seen in arowanas, is caused by keeping a fish in too small a tank. An 8" oscar needs at least double the size tank than what you have him in. These cramped confines will be causing growth problems such as gill curl, and that's just the external problems, you can't see if its causing more damage internally.

Get a much larger tank (at least 75 gallons) for the oscar or rehome it someone who can house it correctly. The damage may already be done however and he may not recover fully.