Thought about G.G.'s Log on photography here quite a while ago. That was a good start.

If one has a small tank and a piece of glass which can be placed in the tank, move the glass about 1/2 inch from the front of the tank. Include dark gravel. Use clothspins to hold the moveable glass in place. Insert fish.

A piece of glass painted with flat black paint can be insered behind all that for a background.

A hotshoe (flash) on a wire to the side helped.

Nobody just held the camera. Most rested the bottoms of their hands on the table or set it on a stable object.

The particular fish in the photo tank was remarkably calm. We asked "killikarl" Doering why that fish was so laid back. He smiled and noted that he put 4 drops of a commercial tranquilizer for shipping fish in there. (I slipped the bag with the fish I brought to photograph quietly back in my fishbox.)