Krylon fusion is safe once fully dry, fully fully dry.
Also I would only paint the exterior of it so as little as possible. you need to put several layers and let it fully dry in between or it might flick away if it is too thin. It must be very dry between coat. I did my overflow as well but only the piping that goes into the sump since my overflow box was alredy black.
Some other experiences from people. Keep reading the thread...many people use it inside the tank without problem, some for 10 years.
http://www.thereeftank.com/forums/f6...ts-138465.html