My LFS actually advised me to set the thermostat on the heater to around
20C - this was to compensate for inaccuracies within the thermostat
itself...to prevent the tank from going too low if the ambient
temperature did...and to prevent overheating if the ambient temp was too
high....in a heated house that rarely falls below 70F and bearing in
mind the temp added by the lighting this kind of made sense to me - just
using the heater as a fail safe....of course I do not live in an area
where temps go that low for it to be an issue overnight...and personally
would not be comfortable myself if the temps in the house dropped to a
level that would really cause the tank temps to drop that
significantly.....some fluctuation is natural though...I was always
taught that it was sudden variations that caused a problem rather than
gradual changes....might be wrong though....

Gill