i don't see why it could be a problem. just make sure there are a few stones or something in the bottom for the gecko to climb out if it falls in. he's unlikely to have a problem, but theres no harm in helping. it'll also help any roaming food items to escape instead of drowning as well.
:Probably an ignorant question but is it safe to have a small pool at the bottom of a waterfall in a cage for a p. grandis. It will be about 4 inches deep max and not very big. I thought I saw somewhere that this was a no go. Anyways, I appreciate any help on this matter.