Just a thought, but aquarium heaters can get very hot. They will kill any organisms that get into contact with them. Once I innocently thought that if a heater would heat water to 23C, they would be safe enough to touch, and got burns on my thumb and finger just trying to pick it up by the glass. They get to at least 100C if not more. But you could run heated water through your solution through pvc pipes, or put the heater in a closed pipe filled with water inside the container, and heat the tea indirectly.
convection! look into it!