Ed & Dan
Please see attached quotation.
JBL ProTemp Basis (250 litre tanks)
Undergravel heating for the planted aquarium
The moderate warmth of the JBL ProTemp basis undergravel cable generates gentle convection currents, (slowly rising currents caused by warmth), which rise from the base of the aquarium. These convection currents ensure a slow and constant exchange of water and nutrients with the upper levels of aquarium water.
In this way, the roots of the aquarium plants receive an optimum supply of nutrients from "seepage sources", just as in the natural habitat. The result is luxurious plant growth. At the same time the ground-covering material acts as one large, biological, slow-action filter.
This is what i have in my tank - bought from Aqua Essential.
Regards
Paul.