Try using a clear Petri-dish/sample container to hold your pondlife/catch and place it over a bit of light green card, it gives a natural looking background and it does not effect the exposure of the image nor will it cast any serious shadows.
Try using a tripod so that you can experiment with lower shutter speeds thus getting the best of any available light, using a rotating polarizing filter helps to eliminate the shine/reflection off the water and try to avoid using flash if at all possible.
Hope this helps Jules.
Steve.
