I have photographed aquarium fish and other waterlife for about 20 years now. It has been quite a challenge to overcome the problems associated with photographing at aquarium shops and public aquariums, where dirty and scratched glass + the fact that the fish don't always behave as you would like. I have worked hard finding my own ways and methods to do this, and over the year I have perfected my techniques and come up with some useful solutions. Here is one.
I started with a 54 ltr tank, where I installed a fixed glass divider, so the fishes wouldn't run and hide in the background vegetation. It worked well but the tank was always an eyesore to my wife, and I was reluctant to empty the tank, just to have to set it up again sometime later. I needed a better way to do this ...
I often photograph at a local fish shop in Copenhagen called Neonfisken (The Cardinal Tetra), and I have a superb relationship with the owners and the people who work there. When some of the employees returned from a successful collecting trip to Peru, we realized the need for a better photo tank, than the portable one they already got, where all the photos eventually looked the same because of the fixed background. As fortune would have it, they had just unpacked a cute little 10 ltr shrimp tank for a costumer and left it out of the packaging - it didn't take me long to annex it into my collections of aquariums. I decorated the tank on the spot, and we were seriously pleased with the resulting photographs. So much, that we did the same when Ingo Seidel from Germany came to visit and photograph these fish.
I have since refined the use of this small tank, by using the top-glass-cover as divider when necessary. I simply put plastic poster-rails on the sides of the glass cover, so it fits perfectly, not allowing the fish to pass to the background vegetation.
Another benefit with the glass divider is that you can successfully have to males of one species on each side of the glass, allowing them to flash their fins at each other without one of them becoming dominated by the other. I have taken some of my very best aquarium photos this way.
When I want to get some better decorated tank shots, where the background and the total layout of the tank is more beautiful, I remove the glass divider and decorate the tank as lovely as I can, or as natural as I can. This way, of course, you run the risk of losing the fish in the plants and decorations, so ways to overcome this are needed. I decorate the tank on a slope up to the back-glass and make sure to plant moss and cut-up algae balls to fill gaps in the rocks and roots, and the plants in the background have their roots pushed snugly between the rocks and the glass, as you can see in the photos. That leaves less spaces for the fish to hide in, and with some prodding and/or patience you can get some stunning photographic results.
Another technique to get the fish moving and doing "interesting" things is to have different species of fish in the tank together. Many fish species act more in the presence of other fish species, and again the resulting photographs can become absolutely phenomenal. Fish like suckerbelly loaches and killis are especially good to photograph this way.
For prodding the fish, I use a natural stick, so if the stick should appear in the photo, it doesn't look too stupid. However, I advise that prodding be done to a minimum, not to stress the fish too much.
So, if you want an easy to install, portable, and very practical small photo tank, this could most certainly be the answer. I bring this tank with me to aquarium shops and to friends who have special fishes to photograph. And when my wife decides it has been out too long, it only takes a couple of minutes to pack down.