Hi Patrick, I'm sorry you went through the expense of a whole house filtration system. A filtration system will eliminate particles, it will not do anything about dissolved salts. To removed dissolved salts you need either an RO system or a DI system, and yes, they are expensive. If you areon a municiple water system the water supplier is required to keep records of water testing and make them available, call your city and ask what the Total Dissolved Solids of your water is, any number under 600 ppm and it is doubtful that your water is the problem.
HOWEVER - I don't think the problem is really the water. You said in your second post you are growing indoors. I think you have a mildew, mold or leaf blight problem. Two common causes of these problems are misting your plants, and not enough air movement. Fungal diseases often can be cured simply by modifying the environment. Get an inexpensive small fan, less than $20 will work. Position the fan so it blows towards or across where your houseplants and bonsai are. You want enough of a breeze that thin grassy leaves would sway in the breeze. Leave the fan on 24 hours a day, seven days a week. All your house plants will be healthier for it.
Second, you need to treat the mildew you got started. Any big box store or nursery will have sprays for Roses. Make sure label says good for black leaf spot and powdery mildew. This will work on most houseplants quite well.