We have heard this complaint from the beginning of time about the brown film from IO and IO Reef Crystals. The type A personality can't handle it in the least that is for sure! LOL! In any event, is your tank brown? I believe not and that brown film is just a biproduct of the initial mix. If IO is impure and this brown film was detrimental to tanks I don't believe Jason Fox would subject his what, 2000 or so corals to it or the absolutely thousands of reefers who use it. And the end of the day this is a hobby, and it's best to make the choice that makes you feel best. Thanks again Randy for your work with the salt mixes!
I can't speak for others, but I don't think it's harmful to your tank - there is an abundance of successful tanks using IO salts, so if it was in some way toxic we should have heard about it by now. My issue is with how it coats everything in my reservoir and caused issues fouling the circulation pump. If it is due to excessive Ca & Alk levels then it's more an indication of a poor formulation. There's no benefit pushing levels so high that they precipitate out.