Poor little sharky. They like warm yucky water, and too much food kills them. I had one for well over 2 years. I fed mine a tiny bit of food, every other day. 

He might have a fish disease called ick. If he has a white spotty look to him,  go to the fish store and get the drops to put in the water. 

Do you use an undergravel filter? I used that and a power filter. Water changes every 2 weeks in a 40 gallon tank, when i steered up the gravel to get the nasty up in the tank before the water change, you couldn't hardly see the fish