I use this single sponge filter in my 10g tank. For my 20g and 29g tanks I use the dual sponge filter. [These are the first photo.] The Hydro sponge filters [second photo] are another type, I have one but find it takes up more space because it has to sit on the substrate (in a back corner usually) whereas the other type attach to a wall of the tank and can be adjusted. The Hagen Elite models may not be available now, but I got similar sponge filters from amazon. The principle is the same. In tanks with plants, and just good floating plants, this is all you need, up to even 40g (I had the dual sponge in my 40g tank for 3 or 4 years before I moved). Essjay already posted about the valves, you want one "free" outlet no matter how many filters/tanks are connected to the pump.