Try shooting in AV mode, set to F20 for depth of field, use manual focus instead of auto, on a tripod if possible, use the self timer or a remote for less vibration and the longer lens might be better but the cannon 100mm macro lens takes some sweet pics if you can get ahold of one. I sold my 100mm cus my tank is 48" deep front to back and it would not reach that depth for close ups on the corals in the back, so I am gonna go with the sigma 150mm. Throw up some more pics when you get a chance-edit those raw/AV mode pics in the cannon program, you'll be surprised how crisp they'll turn out