if u only have a 3 gal and 2 1gals dont bother. bgkf get 20 inches long and can be awfully mean in small tanks, and u cant even safely put a heater on that small of tank.
if im mistaken about the tank size just let me know. it just looks like 3gal 1gal 1gal to me.
i wouldn't add one to anything smaller than a 120 (maybe a 90) and still only one.