If you have killed that many water heaters in that short of a period of time, I think I would be taking a water sample to have an analysis done on it. My thoughts are that you "city water" is killing the water heaters. Where we live, the city water has a "very" high calcium content. We have put two filter systems in before the water reaches the toilets, sinks, showers and water heater. Even with the filters there is a constant battle to get rid of it.
