These Pro-line Aqua heaters offer ultimate flexibility in that the element is submersed in the pond water (no added flow restriction) and thermostats can be used independently.

This isa listing for complete units

Depending on the minerals in the water, minerals may build up on the heating element. Remove these scale build up periodically to ensure optimum heat transfer.
Caution, before you remove the scale build-up be sure to disconnect the control box from the mains and wait at least 30 minutes to let the heating element cool down