Rubbermaid container affecting chemistry?

[nosmok]

I use the 34 gallon brute containers to store my RODI and another to mix my saltwater. Like many of you, I keep a container of premixed saltwater with a heater and circulation pump running, so I have 30+ gallons available at all times. I've noticed over time that the sides of the container get chalky and a test of my stored saltwater show the calcium has decreased to 420 ppm. I've been encouraged by a reefing friend to only mix and use fresh, but some salt mixtures (Brightwell) suggest premixing. Wondering if anyone has experienced this calcium leaching and any other effect of premixing?

 

[Randy Holmes-Farley]

There's no effect from the container.

Most salt mixes are greatly supersaturated in calcium carbonate (as is the ocean), meaning it has more dissolved than it can hold at equilibrium, so it tends to precipitate out. For that reason, you may see some precipitate with many salt mixes, and generally it is not a concern.

 

[PatW]

I have noticed that I can get a fair bit of precipitate with instant ocean and far less with reef crystals. I think the higher levels of magnesium in reef crystals keeps the calcium carbonate from precipitating out.