OK here you go, these couple steps will better yyour tank shots tenfold.
1. Use a TRIPOD, and camera has to be PERPENDICULAR to whatever you are shooting. <<<------ very important
2. Set you camera at the LOWEST ISO setting, 100 preferably.
3. Put your lens on Macro.
4. you want speed around 1/30 and aperature around 11 or higher if possible.
5. set up your timer, or remote and shoot away.
play with aperature for best shots, not speed, unless it's a fish your shooting.
6. Again, Make sure your camera is perpendicular to your subject.