ro is the only way to go. I have mine hooked up to a sterilized plastic garbage can so i can store up to 85 gallons of 15 ppm water. I installed a 966 gph pump at the bottom with a 16 ft. Tube coming off, so i can fill a 5 gallon jug, or a 20 gallon res within a couple of minutes. Depending on your watering needs, you will save countless trips to the store. A good ro system (i use spectrapure) will set you back about $150. The convenience, and gas money savings will add up quick.
Some of my plants are over six feet tall. They are some mothers I have had vegging a while. Im running a RDWC system in five gallon buckets.what size plants do you have? what size planters?
Oh and I think filtered water is better than tap so if you are that straped for cash wouldn't it be better to just get a water filter?