Photographing fish in an aquarium is surprisingly challenging. You would think that aquarium photography would be easy. After all, the subject is literally inches away and confined to a small space. It does not seen to help. The glass on the aquarium is far from the near-perfect (relatively speaking) optical glass of the camera lens. This seems to get some people rather worked up, but I don’t think that makes as much difference as all the other disadvantages. The water itself, moving and being full of dissolved and not-so-dissolved solids affects the light on its own. On top of that, the fish don’t stop moving, in three dimensions. It’s not easy to establish focus, either manual or auto. All the glass around means reflections everywhere. In order to minimize optical distortions from glass and water, it’s best to shoot straight-on. But shooting straight-on at glass tends to encourage reflection (which is why shooting straight-on is not otherwise recomended with glass). The relatively dim light of the aquarium means a flash is best, which increases the chance of seeing reflections. It also tends to reflect off the scales of many fish, producing an unnatural look.
For the zebra nerite snail photograph, I did the following things. First, I used a macro lens, the Micro-Nikkor 60mm AF f/2.8. With the macro lens, I can hold the lens right to the glass (to minimize reflection) yet still focus on the subjects. Unfortunately, while I love this lens, it is an older lens that does not have an on-lens focus motor. It uses a pin drive which is very slow for tracking a small, moving subject. I took many shots, but this relatively stationary snail came out the best because I could focus on it. I also used an SB-600 Nikon Speedlight with an alcohol bottle diffuser. This is a fairly tall flash, aimed straight away from the camera (the diffuser directed enough light to the aquarium), and dialed down two stops in strength, so I did not have much trouble from reflection of the flash itself. However, I did have reflections of my hands and the camera itself despite the reflection precautions. I was able to crop it out here. Post-processing is important in these shots; reflections, bubbles and debris in the water are common issues.
Incidentally, the white structures below the snail are nerite snail eggs. This snail can live in fresh water, but breeds in brackish water. In a freshwater aquarium like this one, the snails lay eggs but none mature to snails. This keeps them from overpopulating. They are also very good algae eaters; they don’t eat other plants, and they don’t develop a predatory taste like some snails. They keep my aquarium sparkling clean.