The brown stuff is not from impurities. It's calcium carbonate precipitation. It shows up around the sides of the bin because the salt is more concentrated in those areas while mixing (this will greatly depend on how you mix your water). Adding a heat source such as a power head or heater to the mixing bin also seems to aid in the precipitation. Given how some salt mixes, such as Reef Crystals, are very high in both alkalinity and calcium, it stands to reason there is going to be a greater chance of precipitation.
A while back, Randy Holmes-Farley did a detailed write-up on this very subject: What is that Precipitate in My Reef Aquarium? by Randy Holmes-Farley - Reefkeeping.com
A while back, Randy Holmes-Farley did a detailed write-up on this very subject: What is that Precipitate in My Reef Aquarium? by Randy Holmes-Farley - Reefkeeping.com



