I'd aim strong corrective color down into the tank without a cover on it angled a bit front to back .. scrub down thoroughly the glass on both sides before and after some intense water polishing ( diamataceous earth and so forth) . Substrate is going to make a big difference in the color of the water due to internal reflections even off the top water surface. Auto-focus is going to tend to be problematic and I'd lean towards manual focus , but If your camera can handle it , great. My pocket olympus did just fine. A fast lens should be a help, maybe just rent one. Anyway, I personally would be thinking it might be cool to use go-pro with wp-housing and corrective lens to either leave submerged a while or just make some videos of the tank and fish as they are now. I had tanks for many years and though I have some pix of some fish and configurations it would really be cool to have a clip of my oscars or corals as a screen saver ,, but I wasn't into photography back then. (nice cichlids though , I always planned to make a false 'salt-water fish' set up with them , but never did. They told me the trick was either give em plenty of space or plenty of company so that one one fish couldn't dominate so readily.
