Dirty Salt?
 

This is my salt water mixing bin. 4 years of mixing salt water with RO. The bin is covered always. Anyone else?
http://img.tapatalk.com/d/13/05/29/ejegegar.jpg

I tend to wash the containers with tap water after, but i only need to make 10 gallons at a time :D

weird..

I clean mine out in the spring and fall. Can't get the bottom clean but it's not as bad as yours. Was pretty grimy though, even my RO water bin needed a scrub.

Salt residue is usually a bit of Calcium carbonate precipitate and if it is brown colored then it is usually some ferric oxide present which usually comes from the Calcium chloride source the manufacturer is using. Ferric oxide will usually settle out on its own, and you can help prevent the Calcium carbonate precipitation by mixing the salt into cold or cool RO/DI water rather than previously heated water.

My mixing tank looks like that too. I took a razor blade and shop vac to it a couple of months ago and got a fair amount of the brown gunk off but wasn't able to get it all.

Same same, dont sweat the small things :-)

I found when I was using h2 ocean and instant ocean I had the brown scum form on my water change tanks. Using reefers best I don't see to get it any more but I changed tub shapes and cover them to keep the light out entirely now so might play a part as well

Reef crystals was always brown like that for me and h2ocean is no different. Just sticking my refractometer dropper into the tank for a second leaves it coated with the stuff. I don't worry about it but it looks nasty.

I blast my reservoir out with a hose maybe once a year or so..

i know a few runners that do that as well before a race to avoid any situations out on the course.