What can influence a tank's Mg reading

[Sebae again]

It could be going out the skimmer if you are not dosing to make up what you throw out. The big assumption with doing regular water changes is that water changes will make up for what is lost,so if you had a fish only tank it would work out as long as the fish never drank any water. The water changes only,with a good salt, make the new water on par with natural salt water. It you do only a ,10-20% water change it does nothing for the other 80-90% so every day our values become less and less. Unless we dose or do 100% water changes we will never win.