What would cause Mg to increase

[rock_lobster]

To what degree are Mg readings effected by salinity?

Salinity wouldnt change in the tank unless adding saltwater as your topoff but then everything would be dead pretty quickly.

 

[Randy Holmes-Farley Verified Community Expert]

Salinity wouldnt change in the tank unless adding saltwater as your topoff but then everything would be dead pretty quickly.

But if he lets the water level drop by evaporation and measures when at the low point, it will impact readings relative to the high point in water level.