You won't harm the plants at all if you mix with r\o water... I personally think it's a fairly good idea in fact! Do you know the PPM on your tap water? Anything over 400 and you should consider using some sort of filter and\or mixing with r\o water. You can buy 5 gallons of r\o water for about 2 bucks at most grocery stores. I bought myself an r\o system, but prior to that I was mixing the water as well. If your PPM isn't too high though, like stinky said, don't even worry about it.