every night or almost every night my oscar will go completely limp pretty much. he floats around on the top of the tank sideways or angling sideways. either floats on the top or sits perfectly still on the bottom moving nothing but his gills. i am worried on what is wrong and he only does this at night, he is perfectly fine during the day but almost every night he does this. if anyone can help me please let me know why he does this, what would cause this, and how to prevent this.