[mike31154]

I managed to find & download the spec sheets for Marineland Perfecto tanks and the data is as follows for their 55 gal. (different specs for standard and those with corner flo, makes sense since corner flo is already drilled).

55 Standard - All panes are tempered

55 Corner-Flo - Sides and Ends are tempered

I have a Hagen 77 display which I plan to use as a sump in the future so I looked into that and the FAQ page on the Hagen site confirms that none of their tanks have tempered glass, but that's a moot point since sphelps has already mentioned Hagen does not make a 55.

So unless you can find out the manufacturer, you're still kind of at square one, dunno. I did have a link to a site describing how to check the glass with polarized lenses, but can't find it. As mentioned you need two polarizing filters. One way to do it is use polarized sunglasses and any LCD screen, even a cell phone might work, although a notebook is better with larger screen. Virtually all LCD screens have a polarizing filter to reduce glare and make them more readable in sunlight. You put the LCD screen on the other side of the glass you're checking. Make sure you put a 'white' screen on the display. Put on your polarized glasses and look at the white screen through the glass. You'll then need to rotate the device with the white screen through at least 90 degrees, possible 180. If no obvious pattern becomes apparent during this process, the glass is not tempered. If horizontal lines or some other pattern appears, it's tempered.