Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I could see using a coffer cup heater in a bucket or garbage can as a heater for water prep. This is a short term use, then you put it away. Start with the water pretty close to where you want it, then use the coffee cup heater to be sure it stays until you are ready for it.
This implies that you are there, with it, so if there is a failure you can take care if it immediately.
I do not know if they will hold up to the many on-off cycles that would happen when they are in constant use in an aquarium, whether located in the tank itself or a sump. I sort of think of that thing as once a day or less often sort of use, and then only for a few minutes.