Unless the fish you plan on purchasing are under 4", you'd be better off with a 20g. Even a rubbermaid tub of at least 20g will do the trick. You can use your water to seed the qt if you wish to have it up and running all the time, but you will want to take out any liverock when you actually medicate. A sponge filter works rather well, cheap to run, easy to clean, and can be seeded by containing it in your sump. PVC
pipes, which are inert, can be used for hiding and separation areas.