Randy, What type of impurities are you referring to? Is it more common in mg chloride or mg sulfate? Does it matter what grade it is? Industrial vs pharmaceutical? Why can my system allow it slowly? Is it not the Sam amount of impurities accumulating over 3 days? I once read it was ok to raise mg 100 ppm per day is that to much now?
I am referring to anything that might be in it. Iron, copper, cadmium, lead, ... In a chemical supplement, almost any ion will be there to some extent. I've measured such impurities in calcium chloride (Dowflake, ESV, Kent, and Warner brands), but never in magnesium supplements.
Some bulk sources of magnesium are also known to have ammonia in them. It is a main reason that some people rant against my recommendation of using MAG flake to supplement magnesium, although it has little ammonia, some bulk brands may. But if you add it slowly, it is not a concern. That use is very different than making a salt mix where you are adding all 1300 ppm instantly.
So the slower you go, the less likely there will be any problem, but the 100 ppm per day that many mention has been shown empirically to be fine for many people. Folks treating for bryopsis add a lot more than that and don't have an issue, although the fact that it kills bryopsis, possibly with an impurity, suggests that there are potential concerns.
So how slow you want to go should depend on what you are using (better grades are less risk, IMO) and whether you have used it previously with no issues. If you up it 20 ppm per day for several days and have no problem, the next time you need it, adding 40 ppm may be a fine plan. Just start slow with any new chemical supplement.
In addition top going slow so you can stop in an emergency, there are a variety of reasons why adding the same toxins over a few days is better than adding them all at once.
Ammonia can be processed by the tank bacteria to prevent it from rising too high.
Many metals will become bound to organics. Copper for example, is bound by organics in seawater. That tends to reduce the toxicity. That is well known, but Ron Shimek rediscovered it when he saw that copper in established aquarium water was not nearly as toxic as if one just added copper all at once to a salt mix. So adding such metals slowly keeps the more toxic "free" metal concentration from rising too high. It also allows them to precipitate or be taken up by the many systems in the tank (bacteria, macroalgae, filters, skimmers, etc.), potentially limiting the toxic concentration.
