do you supplement your 15g SW to 1400 before changing?

Hmmmm, I also question this process... Water evaporation doesn't remove salt, so if you top up with salt, you would be adding more salt. So I am a little confused that your Salinity doesn't change for the worse.

I would personally not do that, as I think it will eventually catch up to you, quicker than you think. Unless you have some sort of salt monster that eats salt.

In regards to MG, 100ppm in a week can happen, I have seen it on my tank. During times when I see more growth than normally, that is when I use more additives

Yes I do, I supply 3 tables spoon on my 15g SW and i am using IO reef crystal salt.

You just need 3 tbsp of Mg to supplement 15g? Your IO mix must be way higher than mine out of the pail. Highest I seen has been 1100 so using this calculator and MagFlake looking at 5oz for 15g.

yes, i got 1300 with my reef crystal so i need 1.6oz to make it 1400

what brand test kit are you using?

I use Salifert

I will try to mix 1 cup of Mag flake with every 5 gallon FW top off and see how it goes

Thanks for everyone input