Great shots AS. I haven't tried my hand at dry land tank photography yet (need some fish in the tank first) but I have photographed a lot of fish. Here are a few thoughts: Unless you are highlighting the mouth/teeth, be sure the eye is tack sharp in focus. In head shots try to get a slight angle. Watch your background. Keep extraneous stuff out of the image. Sometimes you can do this with manual camera settings to give a black background.
I am sure this is hard with tank images and maybe a tripod with the camera aimed at a specific area would help, waiting for the fish to come to you. (Often a technique underwater.)
BTW Happy Halloween.