I am looking for a larger tank and plan to buy an adjustable heater for it. I rescued three bettas they have varying issues so they are in small hospital tanks at 76 degrees. They are active and don't seem bothered by the temp. I don't really want to buy more heaters that will just sit around a month from now but would prefer to fill a larger rubbermade container part way with water, heat it and put the small tanks in the container to keep them warmer. Will this work? If I do this I can just buy the heater that will go in the larger tank where bettas will end up at.