I think its glass. However, you can use glass. If use prospect have to be alot thicker. The pipe depends on your flow rate of return pump. Usually it recommended by pump maker. As long as you install a half time bigger size pipe than the pump output flow pipe, its OK. Please bear in mind that overflow needs more space so your hand might need to get in if anything happens. And pls becareful drilling. Glass drill bit is different and even an experienced tank maker will not have confident on not breaking. Furthermore the tiny bits of glass are dangerous.