Can anyone recommend a good heater for a 5 gallon tank? I'm not too sure how they work, I know some heat up to a certain temperature and stay that warm, other have thermostats that will trun on or off depending on the temperature. Most heaters I see are rated for 10-20 gallons. If I were to get one that you submerge and set at a constant temp of say 78 degrees for some Tetras (I would plan on having 7 in a 5 gallon with nothing more than maybe a betta in there) would this be good?