Answer: In the previous
Halachot we have explained that one may not move a candle or any object that
serves as a base for it on Shabbat. It is likewise forbidden to move the “base”
objects even after the flame has been extinguished unless one has explicitly
stipulated to do so, as we have explained.

Regarding the door of a fish tank
which has light bulbs attached to it, it seems that when the light bulbs are
on, it is certainly forbidden to open the door, for by doing so, one is moving
the bulbs which are certainly forbidden to be moved on Shabbat. Similarly, one
may not open or close any other door which has light bulbs attached to it on
Shabbat.

It therefore seems that there is
no room for leniency regarding this matter unless the bulbs are off. If the
lights will be on at the onset of Shabbat (the twilight period of Friday
evening called “Ben Ha’Shemashot”), one should stipulate explicitly
before Shabbat (once annually) that the door or lid of the fish tank should not
be completely Muktzeh when the light bulbs attached to it are off. It will then
be permissible to open the door or lid of the fish tank in order to feed the
fish, as we have explained in the previous Halacha. (It will likewise be
permissible to do so even according to the Ashkenazi custom, for opening the
door of a fish tank is considered moving Muktzeh for innate usage or for that
of its space.)

Nevertheless, regarding light
bulbs common nowadays which are called “LED bulbs” which do not contain a
filament (a wire which combusts as a result of the electricity), there is room
to permit moving them on Shabbat even when they are on for such a purposes as
feeding fish, for failing to feed them would result in the prohibition of
causing pain to animals. (Regarding fluorescent light bulbs, since we are unsure
exactly how they operate, we cannot issue a definitive permissible halachic
ruling on this matter.)