Do you filter your salt water mix before a water change?
I use Reef Crystals and usually make 15 gallon batches for water changes. I circulate the water with a power head and aerate with a bubble stone for at least 24 hours prior to the water change.
In the past, I would always have a brownish scum stuck to the tote at the water surface at the end of mixing. I also always had this brownish color to my sand bed surface both in the DT and in the refugium. Kind of like diatoms but not quite.
Came up with this DIY filter for use when making my batches. I've been using it for about 2-3 months, and my sand bed is "brownish stuff free". I would also say that my WC water is much clearer than before filtering it. It's easy to make and pulls a lot of "crud" out of each batch. If you haven't filtered your salt water mix, you might be surprised at what comes out. I have not tried this with other salt mixes.
Take a piece of 3/4" PVC and cap one end (longer the better, but a 6-8" piece should work. Mine is around 12"). Drill 6-8 1/4" holes in the pipe near the cap.
Cut a filter sock (or similar fine filter cloth) in a strip wide enough to cover your holes and zip tie it to the PVC pipe making sure the holes are covered and the zip ties "seal" the ends.
Place the pipe in your mix bucket with the air stone in the pipe. Make sure the open end of the pipe is at the water surface or slightly under. The air bubbles will rise up the pipe and create a chminey effect similar to an undergravel filter. It will pull the water through the cloth and up the pipe. With that, you get both aeration and filtration in one step.
Here's a filter after one 15 gallon batch mixed to 1.026 and aerated for 24 hours.
I just recently read another post about someone else getting the brown stuff from RC. I had it before also....reason? Adding the salt into cold water to fast.
Now I don't know if the above reason is true as I'm no scientists but....I now add my salt slowly to RO/DI water that's been heated and allow it to mix using a PH for 2-4 days before using. No more brown stuff!
Solubilities of most salts do increase with temperature, but we're not talking significant differences between tap water and tank water temperatures. The most likely precipitate (CaCO3) is one of the oddballs and is actually less soluble at warmer temps.
Addition rate might be a factor. How slow is "slowly add"?
My guess is you're seeing the effect of 2-4 days of mixing and the "brown stuff" slowly dissolving or getting beat up by your powerhead until you don't see it any longer. But that's a guess...
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I have found personally when I add my salt slowly to the full amount of water and a pump circulating I get no residue. If I add the salt first and pour the wate rin it flashes and all heck breaks loose. If I dump the salt in the full amount of water its a little better but still get some flashing and residue. Pour it slowly cup by cup and let it dissolve and no issues at all. I have also found Oceanic dissolves much quicker than the IO products and Red Sea I used to use.
Certainly not scientific but my personal experience over the last 35 years.
DIY Salt Mix Filter
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