GE silicone 1 is what I have been using for to build my DIY polycarbonate sump. If you look on the back of the silicone under Instructions for Use it even says aquariums on there. If you don't believe me go to a hardware store and check it out yourself, just don't tell stores what you are using it for or they might want to raise the price. That is what the lady at the dollar store told me when I was getting some of my pot scrubbers....