There are better alternatives.
I'm not sure if the real oldschool heaters are still sold in the US:
I started using them in my 10 G tanks, for I didn't want to waste precious space with bulky and unsighty thermostat-heaters.
I have loved them ever since. They cost me like 2-3 bucks, and come in 5 watt increments, from 5 to 75 I think. I haven't needed more than 40w.
It takes a little initial monitoring to see how it handles the job, and determine if I need to go up or down one size. In the summer I run them on the light timer, and get like a 1-1.5ºC warmer noon during the day. Have never had any problems. They also can be run safely inside the smaller AquaClears with a very minor sponge mod.
I don't know why don't more low-tech nano enthusiasts use them.
They'd work well with the pid controller you mentioned.
Cheers.