Even the polarized lense and phone screen trick isn't foolproof. I've exploded a few panes that seemingly passed that test. One was inside... in my living room... 7 years ago... and I still find the odd tempered glass nugget under something or 7 feet high on top of something. But I collect a lot of flee market and Craigslist glass of unknown origin. "Most" large fish tanks only have the bottoms tempered. On your 125, if you can't find who made it and when to call them, you can use both the polarized trick and assumption based on thickness. A tank that size made of 5/16 or less is undoubtedly tempered. 1/2 will be float unless it is a braceless custom build and 3/8 could go either way but most likely float unless it was a custom order, low iron glass could be your giveaway there. Those assumptions are based on a 72x18x21 125. 55s and 75s are the same height but those thickness vs tempered guesses dont apply at all. Just never cut or drill an unknown piece of glass you aren't prepared to replace.
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