Hello; Back in the 1970's I had a homemade 125 gallon tank with an angle iron frame. It also had a tihin metal brace across the top of the tank at the center. The cross brace was held in place by small machine bolts. The tank held up for many years. Eventually enough rust formed betweet the small bolt and the top of the glass at the back. The rust put pressure on the glass and the back panel cracked all the way to the bottom. Lost a tank for not using stainless fasteners.
It occurs to me that there has been a serious amount of cold here in TN in the last week. Could it be that the basement got colder than normal? With tank heaters keeping the water warm there may have been different rates of thermal stress if the ambient air got much colder than normal. A metal frame might have expanded or contracted at a different rate than the glass.
Was a heater near the location of the crack?
That's another good point I've never thought of...makes sense though.