> From: STEVEN R AMOR <n1036416 at student_fit.qut.edu.au>
> Question2:  I've almost finished building my DIY Under gravel heater.. 
> It's a lowish wattage, so I guess it will be staying on most of the 
> time.  I've built an electronic temperature controller, bnut I'm not sure 
> what the literature says about how much hotter the gravel should be than 
> the water itself.  I presume it is only a small temperature increase, but 
> does anyone have any suggestions? 

In our "High Wattage" Discus tank, the bottom of the gravel near the
coils is 10 F above water temp (gravel = 92 F, water = 82 F). The
gradient in the gravel varies from 92 F near the coils to 82 F at the
top of the gravel.

In our "Low Wattage" Rainbowfish tank, the gravel near the coils is 80
F with a water temp of 77 F.

Both setups are growing plants very well, with the high wattage tank
perhaps doing a little better.  I can't say if it's the higher heat or
other factors since the tanks are not identical in all respects.