Hello Christoph,
Tom Hayes showed me a good way to heat up a terrarium. Simply
use incandescent lighting to suppliment the fluorescent settup.
perhaps only one bulb would be needed for the desired temp range.
There are some "grow bulbs" (which aren't really that good for plants
but are better than say a standard 60 watt.) that can be used. They
have a mirror in the back of the bulb to radiate light (but mostly
heat) forward and cast sort of purplish color in the light...
He uses couple to keep his D.petiolaris tank in the 100's for
day-time heat.
Dave Evans