The only way I know to find out is to check if the glass is polarized and that requires special glasses. When I got my 30T drilled I wasn't afraid of it shattering because it was a small tank now on my 240 that was a diff siutation ! But I didn't drill the bottom. There is a guy in Houston that can help you out if you send him an email here's a link http://www.wetdryfilter.com/drilling_holes.htm Putting your holes is all in preference, I've got 5 holes in the back glass of my tank 3 in the middle for the internal calfo drain and 1 at either end for returns