To be honest, I have abandoned the Dupla policy of in-tank undergravel
heating because it is cost prohibitive. I bought an inexpensive and reliable
heating matt for use under reptile tanks -- a thin sheet of clear plastic
with heating elements through it--which I connected through a dimmer to
control temperature...never have to worry about shorting out, no worries of
electricuting the fish and you never disturb the coils when you gravel
vacuum...plus it's about 1/10 as expensive. You may wish to seriously
consider this. It is working wonderfully on my small 20 gallon tank. I had
it under my 40 gallon tank, but due to logistics (the design of the stand)
was unable to shove it under the tank securely.
If you saw my cryptocoryne growth in there without CO2, you'd consider this a
reasonable alternative. It's extremely even heating, it is such low heat
that it poses no risk to the glass bottom of the tank and the root growth
I've got testifies to the fact it is certainly worth the extra $25 investment
I made (plus the dimmer).
Try it. You'll like it.