Building pond fountain from rocks

I have a bunch of flat rocks that I have fashioned into a sort of
fountain that allows the pump/filter discharge to spill over the rocks
and into the pond. They are mostly flat rocks and for now the water
runs over them and back into the pond very well. I am afraid, however,
that an animal may disturb the rocks and alter the flow so that the
water does not return to the pond. I would like to secure and seal the
rocks so that they are less likely to be moved or disturbed. Is there a
mix, like cement, that can be safely used to seal these rocks? This
pond has goldfish in it so I can't do anything that would harm them.