Calcium issues?

I have 2 10 gallon betta tanks. Same place of purchase, same temperature.. one tank is 1 year old, water is and always has been clear as crystal for the entire year. The second tank is 6 months old. My issues tank. I have gone through 2 heaters in this tank, due to calcium build up from within the heater tubes... I have so much calcium in this tank, it collects on the side of the tank, and I end up scraping and chiseling it once a month. I am currently on my third heater in this tank. I have baby mystery snails in this tank, so I am not heart broken about it for the health of the shells for the newborn babies. And it doesn't seem to bother the betta at all.. I resent that I lose a heater every 3 months, though.. my question, is... Why just ONE tank with a calcium issue, and not the other 2 tanks, that receive the same water, and the same temperatures, and the same gravel substrate?

Calcuim build up is usually caused when having high GH (general hardness) which I would expect to see the problem in both tanks.

You have a betta in each tank yes and the same substrate, what else is the same and different in the tanks, does one have live plants and the other not for example?

This area is indeed very hard water.. I have to change the elements in my water heater every 2 years..

The tank that does not kill heaters, nor builds calcium on the glass, does have a rooted philodendron plant in that tank. The tank that has the issues only has plastic plants in it. I dropped a calcium tablet in the clear tank, since I have newborn snails in it, and I need them to develop healthy shells.. and I did notice a slight build up of calcium as a result.. I had an issue several months ago in my 25 gallon community tank, which also has a plant rooted in it, but since I cleaned out the water heater, it has not returned in that tank... I suspect my issues come primarily from the hard water, and some sort of coincidence between all 3 tanks.