There are different ones out there that are reef safe. But I once found a web site that tested the effects in the tank, and apparently, the only way to make it 100% reef safe is to hold the rock out of the water for about 10 minutes while it dries. Now, keep in mind, using it in the water wont kill your corals or anything typically, but it does add chemicals even though it claims to be reef safe. I have used it before to glue my live rock pieces together cause my Tessalata kept knocking over pieces.