Hey, I am just browsing pictures of certain lenses, but I thought I would throw my two cents at this picture.
I'm no expert, but when I was shooting digital at fish in aquariums, I noticed that changing the white balance really helped. Otherwise, the camera tries to correct the great colours brought on by using UV lighting.
This is of course only true when UV lights are used, which isn't very often when they are goldfish, but still, it's worth a try. I think it was a tungsten WB.
Experiment and keep shooting. You have a lot of good ideas!
Interesting take on the subject. I think this image would be improved by showing more of the fish - they are your subject but seem to be mostly hidden. Considering this is done through glass - very well done. Good luck in the challenge!