Thanks! and camera is a canon 30D these pics were taken on portrait mode with flash, Im not super experienced with photography so Im not sure of the other details, aperture, etc I'll post up more info on the camera and setting as I figure them out.
Cool. You can have fun trying to adjust the settings and letting more or less light in by adjusting the aperture or by speeding up the shutter speed. Depending on the fish and how fast the move you will get different results. Try without a flash or diffusing the flash (not directly on the fish or through a filter) and see what happens, can sometimes seem more natural as the flash often flares up the eyes or shows a different color. Some lenses allow you to be play around more and give you more range on the aperture.
The megapixel rate of your dslr camera is kinda low for today’s standard at 8.0 (from what I read), most new smartphones have 12.0 or 16.0 megapixel these days for example and most dslrs selling the last five years or so range from 20 mp and up. That said you should still be able to take good shots with what you have and there of lots of things you can do to make the conditions ideal for shooting.
Cleaning glass as someone else said, inside and out, is super important. Also having good overhead tank lighting on the fish apart from the flash will help. Also making sure all your lights around the tank are off, anything that might create unwanted reflection or glare.
Good luck! Hope this helps a bit. Post more results!