Fertilizing in Semi-Hydro Help!

[grasshopper]

I currently use rain water as much as possible. When my rain barrel runs dry, I use filtered water from a Britta water filter that I bought at Target. They come in all sizes and I bought the largest one, which I believe holds just over a gallon of water. I keep old milk jugs, rinse them out and fill them with the filtered water so they're ready when it's time to water. Since I started doing this over 4 months ago, I have noticed a significant decrease in mineral build up in the LECA.