Turn off pumps, and clean the glass. Be sure the surface of the front glass element on your lens is parallel with the glass pane of your tank.
Use a tripod, much better than a monopod for macros IMO. Use a remote shutter release, or timer mode. I like to use manual focusing so I have more control of what is in or out of focus. In our aquaria which are fairly brightly lit, I never have the need to use an ISO of more than 400, unless I am taking shots of fish, in which has I would crank up the ISO higher.