no still use a heater just have it so its not touching the sides, and yeah for a 5.5in, 4.5in and 2.5in getting a larger container like this will be much better than the 10g they are in now, how often are you changing the water? with all that it should be changed like every day, but with the tub just move the filter you have on the ten gallon on it so you dont have to worry about cycling and then add another filter if you have one, in that tiny of a space there is no chance of them getting along, once they have some room im sure they will calm down a lot