It's more likely there was just a weak weld in that area - I don't heat my tanks, but suspect spending time in places like TX, OK and KS in the summer gets them as hot as if I did. Summer before last I saw a month's worth of over 100* days, with max air temps near 120*. The pavement regularly exceeded 160*... There are 100's of RDS-built VO tanks in Vegistroke conversions that heat the whole tank. I've never heard of a leak.

My 200gal tank had some failed welds though. Turns out that 1400lbs of oil SLAMMING into the top and sides of the tank on NJ's finest highways is really, really hard on structural integrity!!