I have the same problem with my media starting to turn green, and I use destilled water, but as you my plants are located on outdoors, so the media get a lot of microscopic living material that will prosper fast.

Im going to get my RO filter... the tap water here isn't too bad, it gives just a little bit over 107 PPM but still I dont want to take any risk... and if on one year I spend arround $119 on desilled water, the option to get a RO filter for $135 plus S&H will make more easy and confortable to have access to good quality water for drink, plants and maybe an acuarium.