You are going to need a way to seal between the aquarium glass and the lens, or complete darkness in the room with only the aquarium light illuminating it's contents. Any attempt to add light from the outside will create reflections (lens flare) that won't be wanted, if you haven't sealed the gap between camera lens and aquarium glass and there is any room lighting at all. Adorama once had a very large rubber lens hood that collapses in a Z pattern, so it can be stored easily, and then pulled open for use. This hood or something like it will make it easy to get your shots, since the front edge of it can be placed against the glass to keep from getting room light reflections (flare), yet flexible enough to allow you to angle the camera for the desired shots and still not have a problem with the lens flare. You can probably find one of these available from a large photography store near you, but assure that it's made to fit the end of the lens that you intend to use.
One of these hoods helps any time that you need to shoot through a glass window too.
I used to photograph my aquariums often. As Harold said flash from above. I'd lay my SB600 on the glass top and adjust power to match the depth of the aquarium. A close focusing lens so you can get very close, or right up against the glass. And of course clean your glass.
I usually press my lens right up to the glass to kill reflections, then use a CPL just enough to cut glare without losing too much light. Fast shutter and continuous AF help with quick nano fish. A mid‑range macro like 60 to 105 mm keeps things comfy without spooking them. My anubias gold sits near the front, and its slow growth makes focusing a lot easier for me.
I have used a rubber lens hood in large aquariums, allowed me to "seal" the lens to the glass wall.
Easy peasy.
In a pinch, you could cut out a bathroom plunger rubber big enough to fit over your lens- it works fine. Been there, done that.
I did a project shooting beta fish in smaller containers. Reflections were difficult. Containers were round/irregular and I wanted the entire wine glass in the image- so rubber lens hood would not work.
Flash disturbed the fish- so that was out.
I ended up with a constant light source directly over the glass. You have to flag it a lot- but once done it works great.
Good luck with your project
gary
