I've always just used plain tap water. Considering what's in the water where I find healthy concinnus in the wild, I highly doubt that anything in our tap water is going to hurt them. (we actually have some of the best tap water in the country)

Yeah, that's right. I'm a bad daddy. Untreated tap water and gold fish. WC and never dewormed, never saw a vet. Poor guys. They only lived 2 decades.