A 2 X 2 X 4 is a 120g. I would get a 30g long tank and make a sump out of it. I have made several sumps out of 30g, 29g, 20, and 10g tanks. The bigger you can use the better. Silicone will hold the plates in place just fine as long as it is 100% and does not say mildew resistant and its clear. I...