I was thinking of a new way to make DIY caves. What I would do is make a mound out of 'GREAT STUFF' sealant, carve to a desired shape, then wrap with fiberglass.

I would think that this would be suitable; afterall, 99.9999% of boat hulls are made primarily of fiberglass.

Fiber glass is fine - just be careful of sharp points and protrusions on it. Sand it well and really run your hands over every surface. You should design it so that there is no place for the water to pocket and stagnate.

Many boat and airplane hulls use a dope to finish or seal - this is not really the best for a small contained body of water such as a fish tank.

I also worry a bit about the GreatStuff - I do not know if any of the stuff can leach out in water over time......You should probably make sure it is completely water tight. It will then be too light, so you will need to weigh it down.........

Many of the really fancy backgrounds that you can buy are fiberglass construct. You can make a clay mold and form the fiberglass over it to make wonderful "rocks" and "caves" just like it is done in many constructed animal habitats.

Wow, a clay mold...I never even thought of that!!!!!

Methinks I have a new weekend project in the works!