The easiest way to avoid reflection of the flash from the glass is to shoot from a slight angle so the reflection goes someplace away from your lens. Make sure you clean the glass well before you start. I find it best to back away from the tank a bit and then zoom in. It is easier to get a good focus at 3 feet away than at one foot and you can use the zoom to bring things back in. Patience is a must when shooting fish. You end up picking a spot where you intend to shoot, take manual control of the focus and just watch fish swim through the area where you are pointed. If something is in focus and looks like a picture you want, you squeeze off the shutter release. If you are persistent and careful about how things are focused, you should get a decent picture out of every 10 or 15 pictures that you take.