I'd be fairly confident that the bottom sheet of a few-years-old 90g AGA tank is tempered. Think of it this way . . . 90g of just water is going to weigh close to 800 or so pounds and I doubt that AGA would make a tank that couldn't even stand up to mere water. Typically, the bottom of larger tanks are tempered, the sides not.
Maybe it's only a recent development, but most tanks I see have a sticker on the bottom that advises that it's tempered.
I have heard that looking at glass with polarized sunglasses may reveal if it's tempered or not, but I don't know for sure. I do know that when I look through my car windows (tempered) with said type of sunglasses I can see "Spots" that are invisible with the
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eye or unpolarized glasses, but I don't know if that's because it's tempered or tinted.
I suppose you could contact AGA and see if they ever made a 90g tank without a tempered bottom.
Of course, one way to check to to whack at it with a hammer or try to drill a hole through it. If it shatters into a pile of small gravelly pieces: tempered . . . if it cracks into large, uneven shaped razor-sharp pieces: untempered. However, for some reason I don't that that's a test you'd like to run.
