Use the cleanest, flattest tank panel possible and light the tank well so you can keep shutter speeds high. A simple setup is white card or paper around 3 sides of the tank, with strong soft light from above or from a 45° angle that doesn’t reflect into the lens. The white surround also brightens the tank and simplifies the background.

Avoid curved glass and, unless you want the effect, avoid shooting through the water surface because distortion is unpredictable. Keep both the inside and outside glass spotless, and clean water helps color and clarity.

For moving fish, continuous/burst shooting improves your odds. Fish behavior matters too: some species calm down after a short time, while others are best photographed in continuous mode.

Several responders recommend not using flash and instead using steady lamps, positioned to avoid reflections. That can help keep the scene consistent and reduce glare. If you do use shallow depth of field, be aware it can hide tank debris but makes focus even harder.

In short: clean tank, strong angled continuous light, simple white background, flat glass, and burst shooting are the main practical ways to raise your keeper rate.