I have a 15 gal. that I bought used with a so-so heater, whisper filter and light. You can set it up only when you need to treat a fish by taking some water out of your main tank, and filling the rest up with newly mixed water. Then, place some gravel from your display tank in the filter. Or you can do what I am getting ready to do; set the tank up as I've mentioned and put a couple of mollies in it and keep it running. From what I understand, mollies are hardy and cheap, and will do OK being treated along with your sick fish. I also am confused about how effective it is to just set up a tank and take your sick fish out of your display tank which has well established biological filtration. Is it not more stressful to put your sick fish in less that optimal water conditions if he is not severely sick? Any thoughts on the subject would be appreciated. By the way, I am almost as addicted to this website as I am to my fish! Good thing I have a laptop, so I can watch my fish while I learn all this great stuff!!