Well, southdown sand says on it "Not for Aquarium Use" too! LOL.
Sorry thomas, I had too.
I have used marine goop in the past on my 300gal in september and haven't had problems yet. I am not saying it is good, but it worked for me and I didn't lose any livestock or any strange after affects, but mine is not submersed. It is on the outside of a wet-dry, on a bulkhead. Some of it is touching water, wherever that leak was, but like I said, I never noticed anything different.