@Nadia Hale - I have one rental and it's in a city that has some of the hardest water in our state. Just my opinion, but also take a look at what hard water can do to your property. Hard water is horrible on appliances such as dishwashers, clothes washers, water line on refrigerators, etc. Not to mention that when you go to clean the place when you have a tenant turnover it will be a nightmare getting off all of the hard water deposits in toilets, showers, sinks, etc. The first thing I did in my rental was install a water softener to mitigate the risks, it was less than $500 for the softener and it took me half a day to install myself. Since it's your tenant requesting it, maybe see if they are willing to go in on the cost or, like others mentioned, increasing the rent a little. I look at it as an investment to help maintain my property, the only thing the tenant needs to do is fill it with salt every few months. Hope this helps shine a different light on your question.