I know this is a bit of an old post, but I thought I'd add an opinion. Now at home, being the green fingered type, I have a propergator for plants (which consists of two large trays approx. 3ft x 2ft with a single large heatmat sandwiched between them...and no this is not my own dodgy design lol) . Now I noticed a while back that it got awfully humid, yet not hot. So I decided to do a little test and put a 12L tank inside of it, closed it up and left it on.
Now granted that it took about 4 hours to heat the tank up, but it did eventually do it and kept it stable at about 25 degrees C.
Unfortunately there was one flaw. Once you took the lid off the propergator the temperature of the tank dropped within 10 mins or so (not good if you did a water change). But I believe that this showed that the right sort of heat mats could be used, if in an suitable enviroment.
It might be an idea to recreate a larger version of the propergator and see if that would work. Getting around the having to take the lid off might be as simple as rigging up a two way water system to do water changes. I wouldn't rule out heat mats completely. Although most of not designed to take the weight of a tank, it might be an investment if specialist heat mats were created for aquaria (if they haven't already been created). Although if one were to go down the route of making a larger version of the propergator design, but for aquatic use. Then I can only dream of the possibilities for breeding gouramis, bettas etc seeing as the propergator I have at home kept the air soo warm and humid that labrynths would have no problem developing at fry stage.
If anyone would like to add any thoughts, this could be a very interesting discussion on future aqua heating design.
