Why not just buy distilled water at the store? It is cheap. You don't have to mist that often, so it will last a while. On my chams' system, a five gallon bucket last well more than a week. Distilled water is cheap, easy to get and store. I would not invest in an RO unit unless I needed a lot of water, like for a frog collection or if I was making saltwater for a reef system. For a single BP, it seems like overkill to me.
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