my current first 2 Oscar have lots of growth difference they both come on the same size 2 and half inch now that fast growth Oscar is 5 inches and the the slow one is at 3 inches, does looks like a little problem on the body which looks like parasite attack, it looks like the part that cover the gills are bitten of by something leaving a U shape but strange thing is it act so fine move around like the others always hungry for food, try to destroy thing, but it's just not growing.

anyone know of this problem please advices.

water tested
0ammonia
0nitrite
5 nitrate
ph 7.6

I assume that's not water problem cos all other fishes is doing fine.

there are times some fishes do not grow the way they should be.... sometimes
genetically, sometimes due to some issues like parasite. as long as he is active
and healthy, you shouldn't worry much.

during my stint as fishpond caretaker, fishes that do not grow much were separated
and try to figure what was wrong with them. we did not just dispose them.

thanks for the information I did not dispose it. since it's size are much difference from the others I move it to another tank afraid the others might harm it as the size are getting bit too much difference.

I found out that Oscar are getting along better than the giant gourami which tried to kill each other of the same species only.