[pogodzib]

Most old tanks are not tempered, so drilling them usually isn't a problem. In fact, most new tanks still only use tempered glass on the bottom. If the glass does break, of which it won't if done correctly, you can replace it with a new glass panel of the same thickness. Replacing a glass side requires getting a new glass panel of the same size, cleaning off the old silicone, and resiliconing the new panel in place. The top plastic brace can usually be removed by cutting the silicone between the plastic and glass that is holding it in place. On a tank that old, it wouldn't hurt to resilicone the joints anyway, esp. if the tank has sat empty for a while since the silicone dries out much quicker when exposed to air. Old, dry silicone is not a good thing when dealing with aquariums. Resiliconing a tank isn't too hard and can save a major leak from happening later. Do a quick google search and you should find some good DIY sites on how to resilicone a tank, or even replacing a glass panel for that matter.

BTW - If I recall correctly, AGA used to use a white label on some tanks and a green label on other tanks. One color meant that the bottom was tempered, the other meant that it wasn't. I don't recall which is which. Another way to tell is by looking at the glass through polarized glasses in the sunlight. If there is a rainbow like wave in the glass its tempered.

BTW2 - Larger tanks are easier to drill than small tanks. Since the glass is thicker, there is a lower chance that it will break due to the added pressure caused during drilling.