You could run an experiment pretty trivially to determine if the hardness of the water is the issue. Get one of those charcoal filters, get some coffee filters, and cook some rice with various combinations of the filtering.

The previous place I lived in had very hard water, and I found that running the water through both a coffee filter and then a Brita charcoal filter worked rather well to soften the water and improve the quality of the rice I was cooking.