Should I add anything to the water to help clean dust etc. from the interior?
To clean a tank, you can use a vinegar + water solution. That should be able to clean any dirty areas. That and elbow grease.

How can I tell if the glass is tempered?
Well hopefully there is a tag on the tank (hidden on the trim perhaps) that tells you which brand the tank is. Then is a matter of getting ahold of the manufacturer and see what they say. Otherwise I'm not sure there is a way of determining if the glass is tempered or not.....unless you start drilling and the thing either shatters or just cracks ;)