Don’t use a hanging aquarium heater ( a long phallic looking
tube which is designed to hang on inside wall of tank, usually
6" to 18" long) unless you can cover it with solution to within
1" at most, 1/2" if more than 20 degrees F difference between
air temp and solution temperature. Be sure to let cool down in
solution. Fish tank heaters will bring temp up slowly and hold
solution at whatever temp you want; a submersible is a bit
more expensive but wouldn’t have to have solution 8" deep or
more. Main problem is size - heater will shatter if heating
area not submersed. I don’t have/never used ultrasonic, but
have kept fish all my life. I’ll try to find article on under
gravel heater that you can make yourself if you’re interested -
heat solution from bottom with wires. Kat