If you want to drill a tank, then the glass needs to be plain glass. Temered glass is plain glass that has been reheated and when it shatters, it breaks into small rounded pieces instead of long jagged pieces. Tempering glass is a safety issue. It is very difficult to drill glass after it has been tempered and will void any warranty on the aquarium if you do that.

So if you are going to but a tank and drill it yourself, you need to drill the glass that is not tempered (which usually means just the sides - you have to check with the manufacturer to see if which sides they use tempered glass on) OR you will need to buy the tank pre-drilled. What the manufacturer does is takes a plain piece of glass, cuts the holes and THEN tempers that pieice of glass. They then put the aquarium together and send it to you. Hence these tend to be more expensive than a regular aquarium that hasn't been drilled.

Your other option is to get an arcylic aquarium. These can be drilled by you without any of the associated problems with tempered glass.

HTH