Silver Shark Not Well

more information would help. do you have a picture of your fish or do you know the scientific name for it? there are a lot of fish that could be called Silver Fin Shark.

What size is the tank?
How long have you had it set up?
What fish do you have in there, how many, and how big are they?
How big is the shark?
Do you change the water regularly?
How much and how often?
What are your exact readings for ammonia, nitrite, and nitrate?
Have you added any new fish recently?