Sub heaters are about the best thing you can do for a planted tank;
- even temperature for a given depth
- accelerates the decomposition of the mulch
- allows plants to absorb the mulch faster and thereby promotes plant growth
- the water convection through the sand provides a form of filtration
- it is widely accepted that plants like "warm feet"
- sub heaters are underpowered so as not to damage the plant root system through overheating
klayus,
if you are attempting to even the temperature throughout the tank then I would think again.
In nature there are natural thermoclines in the water. These are inversion layers where one layer is at a different temperature the other. Fish move through these layers and use them to find comfortable temperatures. Not all fish like exactly 26°C, this is a best fit temperature.
Regards
Andrew