Dangerous Ornaments

Hi,

I want to create a functional ornament for my arquarium which houses two
goldfish. What I want to do is glue various seashells to a plastic or metal
tube. The ornament will be placed in the corner of the tank to hide an
air-stone and air-line. My question is what material and glue can be used
that will not affect the health of the fish. I was thinking of using a 6
inch (length) plastic tube, like the the kind that plumbers would use, but I
have no idea about the glue. I want it to be non-toxic to the fish, but be
strong enough to hold the shells onto the tube base.

Has anyone attempted something like this before? Any ideas on how I should
go about making it.

Thanks