I'm sorry you lost your fish.
If other WC were done with no salt, it could in fact be that you were using un softened water, which would likely not have been as big an issue for your fish.
As to one of your questions, the water softener removes a lot of things, so not using it may be an issue. However, just in case, it's not always true that water softened water is routed everywhere in a residence. It's typically sent to the laundry and the showers (due to how soap interacts with it), but sometimes not to the kitchen. The reason is that some people have health issues with water that has been softened, so kitchen water does not get softened.
If you were not getting your WC water from the kitchen and if you have been getting buildup of lime/calcium on your kitchen pots/glasses, then there's a good chance that the kitchen is not getting softened water.
If other WC were done with no salt, it could in fact be that you were using un softened water, which would likely not have been as big an issue for your fish.
As to one of your questions, the water softener removes a lot of things, so not using it may be an issue. However, just in case, it's not always true that water softened water is routed everywhere in a residence. It's typically sent to the laundry and the showers (due to how soap interacts with it), but sometimes not to the kitchen. The reason is that some people have health issues with water that has been softened, so kitchen water does not get softened.
If you were not getting your WC water from the kitchen and if you have been getting buildup of lime/calcium on your kitchen pots/glasses, then there's a good chance that the kitchen is not getting softened water.