Is standard silicon sealant safe in a fish tank?

I had a problem with waste gathering between a black plastic filter/
pump cage and inside edge of glass so removed the cage and refitted
with standard sealant leaving no gap for water/waste to get between
cage and glass.

Silicon was fully cured before adding water, fish lived happily for
over 6 months but recently had problems and noticed slight oily film
on water suface.

Could this be down to silicon all this time later? could it be
possible that various tank cleaning fluids react with the silicon?