Midnight

I am pretty sure than typically all 55G tanks are tempered on all sides.

My understanding is that most manufacturers only temper the bottom. The sides are generally not. If you know the tanks manufacturer, I would contact them before drilling. I have drilled a standard 55 in the side panels and not had any issues.

Usually only the bottom of tanks are tempered...55's for sure in my experience.

Can't speak for yours, but I drilled a couple of them on the sides and never had a problem. I've drilled 19 holes in aquaria successfully and only one unsuccessfully so far.

i just did a 55. the sticker said the bottom was tempered so i went through a side. turns out it's tempered.
oops. now i need a new sump

It really depends on the manufacturer but after a certain gallonage they tend to temper more sides. 55 is right around that threshold.

Forgive me for linking to other forums but here is a very interesting method:

http://www.salt-city.org/showthread.php?t=9542

Hope this helps.

I know this is old but 55s are generally only tempered on the bottom, there are however times when at the manufacturer they will throw a tempered bottom on a side, I just drilled a 75 a 36 and a 29 all no problems.