Oh, I didn't realize you kept the candle burning. I thought you had just heated the terracotta and tucked them in the towels. The problem with keeping flame going is 1) fire hazard and 2) heat is escaping from the top/uncovered areas. If you have Amazon Prime you can get these. Not sure at this point if the way you're doing it is the most effective. Maybe do an experiment with heating one terracotta up, tucking it into a towel without a candle, and completely covering the tank? Then measure the temperature an hour later compared to your old method? I can't help but think exposing your small tanks to sunlight to warm them (screw algae at this point), then insulating them for overnight wouldn't be better.