Use the cleanest glass possible first—dirty glass will ruin contrast and sharpness. Avoid soaps or chemicals if it’s a home aquarium.

The main challenges are low light, moving fish, and reflections. Fish can change direction quickly, so use a fast shutter speed if you can. That usually means using a fast lens and taking advantage of as much available light as possible.

Reflections are a big problem. On-camera flash generally won’t work because it reflects straight back from the glass. Off-camera flash can help in some situations, but getting the angle right is difficult. A practical approach is to get very close to the glass and keep light on your side of the glass to a minimum.

Aquarium lighting can also confuse auto white balance and give a yellow cast. If possible, set white balance to Tungsten as a starting point. Better yet, shoot RAW so you can fine-tune white balance afterward for more accurate color.

In short: clean glass, minimize reflections, avoid direct flash, use a fast shutter with a fast lens, and shoot RAW for easier color correction.