Wow, it must be some seriously cold water you are adding, and enough of it to cool the water down 5 to 10 degrees or more very quickly. I have owned several cheap heaters, and still own them today, because they all still work. The only one I've owned that broke was, believe it or not, a titanium heater. After a few months this heater leaked current randomly into the tank (wondered why my fish got so skittish all of a sudden :idea2: ). I didn't own a GFCI at the time, and I didn't find out for at least a month, when I finally felt a "buzzing" feeling when I put my hand in the tank. Thank God, it could have been worse! I never unplug my heaters for maintenance, though. Guess I'll learn the hard way :thud:
If you are adding water that is cold enough to crack heaters, that is the problem right there. If you have non-submersible heaters and the water level lowers below the heating element, you asked for it to crack! You must let those heaters cool down before letting air hit them. Many times they don't even crack until back in the water, though.