I used E6000 on my stacks then went back with a clear outdoor silicone (like a caulking) around the areas that were glued and I had no problems, even with the birdbath.

I think if there is a place to drill a hole in a saucer or whatever is holding the water, that would take care of the problem. That's what I'm going to do when I put mine back together. It doesn't hurt if they get wet, just if they stand in water at the E6000 glued spot.

oh... my choice to stick the aquarium gravel to a concrete walkway/stepping stones would be an epoxy.. one with a thick consistancy... or epoxy grout... it adhears to the slick non porius glass gravel like a gorilla.. gives a bit of edge embedment.. and is absorbed into the concrete of the stepping stones ..I can't remember if it's Lowe's .. or Homedepot that carries it... got a cute one with glitter in it for one tileing job.. and years later I'm still finding it about impossible to remove little bits that were stuck to the top of the tile..
If I remember it came as a mlkey liquid in a plasic bag you'd cut up and add to a pound bag of the grout... ifyou put the liquid into a plastic jar you could mix a bit of it in a small amount of grout.. and save the rest for the next project.. it doesn't absorb water or anything else.. so it's used where sanatary concerns of food or bacteria might be left behind after cleanning... [ kitchen counters and bathroom floors in comercial settings ] and as such doesn't absorb water that might be in the grout and frozen and broken away in freezing.. and the epoxy sticks to the glass quite well... I'll likely be at both stores today.. buying things for mother ... just like my life in the city... where I visit both at lesat once a day... It's civilized heere in N . AL.. S. TN ... where huntsville has multiple locations for both.. as well as two HobbyLobbys.. it's just all so much further away than the 5 minutes it takes me to get to them in the city ..
there's always plenty of projects to do here.. between the garden.. the house.. cattery..kennels..yard and garages..when they are all done in a few weeks each year