I have a 56 gal freshwater with 5 sparkel tetras, 2 clown loaches, 1 neon, 2 jumbo sailfin mollys, 2 black mollys, 1 para pleco, 1 pearl gourami and 1 silver dollar. I would like to creat a cave using slate or shale, or pvc pipe, what can i use to adhear gravel or stones to the pvc for a...