Niki, are you using a tripod? if not i would suggest getting one, i got a cheap 20 dollar one at ritz and works really good. Also as far as settings go use manual settings, play with the iso, normally when shotting bottom of the tank i need a higher iso and when shotting closer to the light lower iso, also the esposure, you may need to set a longer exposure or lower, i for got my current settings but when i get home i cam post them up for you. What camera are you using?
On a cool note, i just figured out that a coral sold to me as a miamy hurricane is actually an emerald mummy eye . its getting a nice emerald color around the eyes and blueish around the edge.