Your correct, It is NOT OK to drilled tempered glass. Just because the bottom has a TEMPERED tag on it, does not mean you should lose hope. Likely it is just the bottom of the tank that is tempered.
Do you know who the manufacturer of the tank is? Do you know the age of the tank? These two answers alone would likely reveal whether or not the tank is tempered on the sides just by visiting the manufacturers website and downloading the PDA document on their tempered tanks. Tempering the sides of tanks is becoming surprisingly more common on new tanks due to the fact that more manufacturers are now selling RR tanks.
If your still not sure about the sides, get yourself two polarized camera lenses, holding one on each side of the glass- you should see a crazy pattern if it IS tempered. Do the same test on one of your car windows (NOT windshields) because they are tempered as well. Sometimes just wearing polarized sunglasses will reveal this effect if your looking at it outside in the sun.
Polarized camera lenses can be had for about $20 each. A little pricey sure, but you would need to compare that price to a new(replacement) pane of glass, and decide what is more valuable in your situation.
Personaly... I would not worry about it and just drill the thing, but I don't want to be the reason you broke a pane of glass on your tank, so please disregard my personal opinion.
