In a attempt to be fair to all parties, I have read through all the posts from the beginning.
I am a reef store owner, I sell only one brand of salt, Tropic Marin Pro Reef. I also use the same salt as my pre mixed salt that I sell to my customers.
I mix thousands of litres of this salt every week. previously after purchasing new mixing vats 1000 litre potable water vats litres. I never had any residue after mixing or storing the slat mix. However after some period of time I started to notice a brown substance that covers the mixing container and my pump over an extended period of time. I also found that the pipework had developed a slime coat, similar to bacterial bio film.
I presume this is the reason people mixing in an alternative vat do not see the same effect, as its a clean vat.
For that reason I now regularly sterilise and clean my containers, RODI and salt Mixing vats.
Once the containers are cleaned, the brown covering does not appear again till about 4 weeks later. I have deduced that this covering is in fact a bacterial reaction. It is also one that gradually becomes more noticeable over time. One thing that I find puzzling is that even with hydrogen peroxide, I can not completely clean the pumps.
@Lou Ekus , although I get my support from Hans Werner Balling himself at the German facility, I have always found Tropic Marin EU to be extremely helpful and honest.
I wish you a happy new year and hope this maybe sheds a little light on a possible reason why the brown film occurs.
Best wishes from across the pond.