I can't seem to get it to focus fast enough.
This is THE problem. First - you need TONS of light. I use a strip reflector at the front of the top of the tank. I use as much ambient room light as possible AND I use a pair of video lights on both sides, being careful not to get any refection by setting them above the water line - aimed downwards. Be sure to set your white balance. AVOID using your camera's built-in flash. All you'll get is a shot of the flash going off. The last step is an inordinate amount of patience. It is not unusual for me to have to take 200 or more pics to get just one useable image. Sometimes it takes twice that. BTW
- I shoot the majority of my pics in "macro" mode and usually the fish is out of the frame by the time the shutter goes off. It's sort of like shooting at a clay pigeon. You have to guess where the fish is going to be a second or two ahead of time. It's not easy - but the reward of a great pic makes it all worthwhile. Keep trying. You'll get the hang of it. - Guppyman