if your turning off all the lights except the tank lights and your using the "auto" settings your best bet is using a tripod, your not able to stand still enough to not have shake which is causing your blurry pics.
On the close up shots scott is correct, turn the macro setting on which is the flower icon, some camers will have 2 settings, my one camera does and you can set the camera right on the subject your shooting its that close.
this is a pic of my cell phone, the lens was resting on one end of the phone at a 45* angle.