I recently got a 200 gallon reef tank off craigslist. The tank was in storage and the plumbing needed work, broken sump, etc.. I wasn't interested in using it as a saltwater tank, as I already have freshwater fish that needed moving into a larger tank. The tank was drilled with two overflow boxes, so my husband just capped the holes with end caps underneath the tank and siliconed the caps. The pipes are threaded but he couldn't find caps that fit correctly. He used GE1 silicone on three of the holes, ran out, bought a different kind of silicone and let it cure. Unfortunately after setting up the tank, the other kind of silicone he used didn't cure. The end cap pretty much fell off and the silicone was jelly. We drained the water below the overflow so no more water could go from the tank into the overflows. We re sealed but apparently some water got left in the piping in the overflow and ruined the silicone before it could cure. Any advice on what to do to get this hole plugged other than drying it out as best as possible again and re capping it? The other three are holding up perfectly fine. I am running three Fluval FX6s on the tank and was considering trying to run a hose from one of filters to this hole but I'm not sure how any of that really works. I've dealt with large tanks but only freshwater and nothing drilled, I've always stuck to FX6's for filtration and could use some ideas.