Not true in my book I have been using plan old tap water in my tank and running carbon in a canister filter for about three years now in a 30 gal sump less and my rock is totally covered lol when it started to grow a bit some guy told me to shut off my filters and brush the coralline with a stainless steel brush and let it settle I did once a week and in about 6 months all my rock was covered completely you just need a good seed to get the ball rolling I'm not saying that is gospel but it sure worked for me
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