Well, there are two kinds of 'water softener'.
One kind actually does soften the water, by removing some of the dissolved minerals. That sort will be okay for you to use with your fish (and be safe for you to drink!)
The other king replaces some of the minerals with sodium; the water behaves as if it's softer, so it's better for your washing machine/dishwasher, but it still has the same amount of dissolved matter, and is actually slightly brackish, so no good for fish, or for drinking.
You'll have to find out from your parents which they've had put in.
If it's the second type, they should have been told it's not for drinking, and you should have at least one tap that's not connected to the softener.