One quick addition to that, I learned the hard way, if you have to heat the tub make sure that you have something to keep the heater off the plastic so it wont melt through.
That is true but with rosey reds or gold fish and bait minnows no heater is needed at average room temperatures, for other fish the submersible heater can be secured to a piece of ceramic with a couple ofhigh temperature zip ties or kept off the bottom by a layer of fine gravel.