mmm..wat is the fish that u are thinking of shooting?
guppy? betta? arowana? catfish?
For fish taking, a few steps is basic
a) clean the glass tank's inside and outside until it is some sort of "transparent" like dat...meaning cannot see dirt etc...
b) switch on the tank lights at the top. Preferably they are strong so that they can light up the aquarium
c) remove anything ugly in ur tank eg. plastic bowls, or rotten fish, scales of dead worms that u feed ur arowanas.
d) hopefully ur water is the clear type, and not those that is milky and dirty.
e) Turning off ur filter helps too. Because when the filter is off, then won't be messy water splashing or going into ur tank water.
After the above steps is done, just go and shoot ur fish. Btw, it is recommended that u get an external flash. With the D80, it can act as a commander. Get a Sb600 or Sb800. the D80 will shoot head on, perpendicularly to the fish tank, while the sb600 or sb800 will flash at an angle wirelessly. Flashing at an angle helps to minimise unwanted reflections.
i believe adding a circular polariser will also help to minimize unwanted reflections. u might want to try this out if u have the polariser.
For fish like arowana, they swim pretty fast, u might have to shoot quite a number of times to get it right. For catfish, u might have to bend down to shoot perpendicularly to it. Because the fish is always at the bottom of the tank.
For smaller fishes like guppies and bettas, they are quite tricky. U might have to use macro lens.
I think thats about it, all these are what that flies into my mind now. I go sleep liao... enjoy shooting....