Electric or gas powered?
If you have a decent multimeter, see if you have any voltage between the tank and a good ground.
Have you ever looked at the anode to see if it was all eaten up? You may need to change out your sacrificial anodes periodically.
How much pressure do you use when blowing down the heaters? (sounds like an air compressor is being used?) Excessive pressure can stress the vessel and may open up cracks.