I thought I was the only one seeing build up in my mixing tank. in 2019 my drum was pristine after several buckets of TM Pro. This year it has considerable gunk built up. Not griping about the performance - it's excellent - just wondering why the crud in my mixing tank. Good excuse to clean the drum.I had a lot of brown-black build-up from my last bucket of T.M.Pro. It seems OK with a new bucket. This is what I was told from T.M. when I sent them a photo. (Note: my tank is in a dark room which would tend to rule out algae/cyano):
"It definitely does not look like any kind of precipitate. It absolutely has the look of something biological. It could be cyano bacterial growth, if the holding tank is exposed to light during a period of holding. Or it could, more likely, be some kind of algae growth. It could also be manganese precipitates induced by bacterial oxidation. This means a biofilm of bacteria covers the insides of the tank. After adding our salts, which contain slightly elevated concentrations of iron and manganese, these trace metals get precipitated and form this dark film together with the bacterial biofilm. Finally this is exactly the mechanism by which both trace metals are removed from the saltwater after adding it to the display tank, so the elevated concentrations from the salt cannot be found in the display tanks.
If we are correct about any of this, it should not show up directly at the time of mixing, but some hours or maybe even days later. A good cleaning of the mixing tank and using of new batches of salt water solution soon after they are mixed, should make it that this issue does not show up again so soon."
Zero concerns about a little PO4 showing up.