[cs_gardener]

Convection occurs because a substance has a lower density at high temperature than at low temperature. This causes the low density, high temperature substance to be buoyant, to try to float in the high density low temperature substance. So, if you can add heat to the bottom of a mass of water the heated water will rise, creating convection currents.

Without a substrate heater, or a heater under the tank, an aquarium will never have warmer water under the substrate relative to the tank. (An exception would be for those using water chillers to keep the tank water below the room temperature.) Therefore, none of us with conventional tanks will ever have convection water currents in our substrate.

This makes sense to me. At one time during the winter I didn't have sufficient water movement in my 29 gallon tank because the filter needed cleaning. I reached into the tank for maintenance of some sort and the upper part of the tank was right at the correct temp while the bottom 6" of the water column was definitely cooler if not outright cold. I cleaned the filter right away and the temperature became consistent throughout the tank again.

I've never measured the actual temp of the substrate in my tanks, but there has never been a noticeable difference from the water temp at the bottom of the tank when I'm digging around with my fingers trying to plant or uproot something.

For me, as long as I have sufficient current in the tank to keep the water well mixed with no stratification I don't worry about the temperature of the substrate. My plants are growing well and I don't think heating the substrate is worth the hassle or expense since I'm happy with the results I'm getting.