There's a lot of info that's left out Don:
- Did you make it and "Cure" it (After casting, did you leave it in a warm and humid place for a while?) outside the tank and left it in the tank with water afterwards?
- Are you using saltwater in your tank with this diy rockwall?
- I am not so sure if anything cast out during the Kure will do anything but give off a high pH, but even still.. why kure it in a tank? My worry would be the prolonged chemistry with the silicone... though I honestly don't know what, if any, problems it would cause just to the tank.