You are giving too much credence to a cheap kit test result when scientific fact says it cannot have risen as suggested (assuming you did what you report).
There is no possibility it was 1200 ppm, then you added 25 mL of the red sea supplement to 20 gallons and observed a rise of 200 ppm, to 1400 ppm.
That is actually impossible.
Red Sea's MAGNESIUM FOUNDATION C - contains blended salts of magnesium and is part of Red Sea's complete Reef Care Program. Magnesium complex for balanced formation of coral skeletons
www.redseafish.com
"
Liquid supplement: 1ml will raise the Mg level of 100 liters (25 gal) by 1ppm."
Red Sea says the rise would be 31 ppm.
EVEN IF YOU ADDED 25 grams of dry solid magnesium supplement, it would only rise by 40 ppm.
The additive cannot possibly have been concentrated enough to give a 200 ppm rise.
Thus, test error is by far the most likely possibility. Nothing else is even plausible overnight, IMO, if you accurately reported what you did and observed.
