seal

As some of you know, I've been constructing a water feature and other features for my new viv. These features are built using foam/cement/paint/sealer. Yesterday I finally got to paint after a couple of months of working on these and spent all day doing it. Today, I am a little deflated. I...

So I have a tank with a couple of drilled holes and I would like to seal these holes up. What do you guys suggest would be the easiest method for sealing these holes. Should I just place a small piece of glass over each hole and cover that with silicone, or is there a better way?