I'd invest in a nice rubbermaid tupperware container for one of them, as many gallons as they need, and shove another heater in there until you can figure something out..
Youre not a bad owner, just misinformed.
The 40% x2 as a compromise isnt enough, sorry. Its like 40% every other day minimum.
I suggest going out and getting an ammonia test kit (theyre a worthy investment, and arent too much) with *liquid testors (strips are not accurate) to see the ammonia levels in your tank.
You could monitor it and make a custom shedual for changes this way- monitor once or twice each day, and when it gets above .25-.5ppm (this is a bit too high, imo..), there needs to be about a 30-50% change, then monitor again, making sure each time you do a change when it gets to that level. The lower, the better.
.5ppm long term is not good for your fish, though if it happens occasionally and is not allowed to get that high often, its alright. 1pmm+ will kill your fish or hurt them badly.
Once you do that for like two weeks, youll become familiar with a regimine suited for your tank.
Invest a bit in stemmed fully aquatic plants, too. They eat ammonia and will allow for a healthier, more comfy environment! (actually, all live plants will, but the stemmed are ammonia nomming machines!)