A charcoal filter does not help rid water of dissolved solids from what I understand. The only methods that are effective are reverse osmosis and distillation. RO systems aren't inexpensive. Distilling the water yourself would be relatively cheap, but a total pain. If you have a small collection, as I do, it may just be better to buy the water. Have you tested the water? If it isn't too high in dissolved solids, you could use the tap water with an organic fertilizer such as fish emulsion, which doesn't contain salts. The PH levels are important here as well...they shouldn't be extreme in either direction. I think that neutral to slightly acidic is supposed to be ideal. Someone correct me if I am wrong please...this is my plan to deal with my water.