Try this on your carbon

[andestang]

Well did a major filter clean out of my Koi filter which of coarse includes changing out the carbon. Did the hot water soak and decided to I didn't need as much as I soaked. So I drained the extra carbon and put it in the lid of the carbon container. Went back later to clean up my mess and noticed this in the carbon (undisturbed) Not just a little rust but lots, even stirred it up to check. And this is just barely a handful. Shows some of the crap that's on the market. Try this on your carbon, I'm curious to see if others find this.