I've never really heard of an auto top off on a 24 gallon. That's a very small tank, compaired to what I am used to. If you have an RO Unit, run a line two your aquarium, connect it to a float valve, and silicone it to a piece of cut egg crate, then silicone the egg crate in place on the back of your aquarium. You can find small float valves at large plumbing stores OR at your lfs.
There are also electronic switches. I have heard those fail after awhile.
hope this helped