It's the tap water. Most municipal and well water supplies have fairly high calcium levels along with a myriad of other dissolved solids as well as metals like iron, copper etc. When using tap water your starting calcium levels can range from 40-100+. Then you add your salt mix and now your calcium can be anywhere from 450-550+. What happens is over time when topping off or water changes, the calcium levels creep up. Before you know it your calcium is off the charts.
You really should get a RO/DI unit. You can pick up a really good one from Amazon for about 150. The one I have is the LiquiGen 6 stage 150GPD unit. It is an absolute beast. My source water comes in at 180-200 ppm and after the DI stage it reads 0 TDS with a PH of 7.0 and ALK 2.0, then it goes through the Alk filter that raises the PH roughly 8.4 and 9.6 with a TDS of 11. It is a dual output unit so you can have one line after the RO or DI stage for filtered drinking water and then the second is after the ALK/Carbon filter for aquariums. When mixing in Red Sea Pro Salt my Cal comes out to 420 ppm per 5 gallons.
https://www.amazon.com/gp/product/B01MS9NM9N/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1
You also will want to get an inline TDS meter, pressure gauge, some extra tubing, and Teflon tape. The supplied tubing is fairly short and they don't include any Teflon. Luckily this stuff is rather cheap. The pressure gauge is a must as you need at least 45 psi for the RO to filter properly. Most municipal water supplies range from 50-60 psi.
One thing to note if you get this model, they don't plumb it correctly after the RO stage. You want the output from the RO to go into the inlet of the DI stage, then to the inlet of the ALK/Carbon. There is a supplied T-fitting that you can install with a ball valve on it either before or after the DI stage for drinking water. From there you would have your PH/Alk adjusted fresh water.