On occasion, I've had to use a tank-divider in one of my fish aquariums to convert it into two tanks. Penn-Plax makes good tank-dividers in different sizes. I have one that fits 29-55 gallon aquariums.
A tank-divider is a great thing to have on hand if you have a fish aquarium. You never know when you might have to convert your tank into two. I had an injured fish once and wanted to keep him separate from my other fish. In went the tank-divider. I also had a few small fish at one time that I was raising from babies and had moved out of the fry tank they were in because they needed room to grow. When they were first put in a bigger tank with the big guys, they were too scared because of their smaller size to mingle with the other fish. In went the tank-divider, until they grew a little bigger. I also had two fish fighting at one time. Again, in went the tank-divider for a short period of time, until the fighting phase passed.
This tank-divider has come in so handy. It is very easy to install in the tank. As it states on the box, the tank-divider will provide a separate area for breeding and baby fish, separates rough fish, and allows for unrestricted filtration and heater circulation. Plus, it saves the stress of having to transfer a fish from one tank to another (unless the fish is sick and needs medication. Then he should be transferred to a sick tank which is set up solely for that purpose).
If you have a fish aquarium, consider having a tank-divider on hand. It is an inexpensive and safe way to convert one tank into two.