I bought 2 Brutes - have my RODI water drip into one that looks as clean as new, and the other where I mix the SW. Over time a lot of brown (diatoms?) developed in that can (and coated the heater as well as pump).
I just realized I have an old glass tank that I will clean and use for mixing SW (assuming vinegar for the heater, pump and tank, right?). I'll see if that helps