Okay, I have 3 fish in a 12 gallon tank and it's only been a few weeks since we got the fish and my cory catfish is fine and one of my mollies is fine, but my other's back is curved sideways and he is sort of drifting with the current and occasionally he would turn upsidedown. He hasn't been eating a ton for a few days, but if you have a suggestion for me to see if he has a symptom, tell me please. This is my first fish and I don't want him to die! Any help will be a savior!