I had a 40g on the porch with two dormant goldfish from our pond in it...well it froze pretty hard around here for about a week, The back of the tank shattered with out me noticing until the water was about 2 inches deep as it thawed.
Figured they were goners..my chocolate fantail and my 6 inch calico subukin...went out to get the dog bowel the other day and glanced at the tank
both on there sides in half frozen muck breathing!...now they are in a bucket until i can go get a plastic tote for them.