Actually I make a dilution with RO water and tap water. This the way I fix my Mg.
As for the feeding is not so much, so after many tests I found out, by adding
NO3 =11 ppm
PO4 = 4.5 ppm
Fe = 0.35
per week I have the amounts of fertilizers I want in my tank.
before the 50% water change, I have
NO3= 25 ppm
PO4 = 1 ppm
Fe = 0.1 ppm
after the water change
NO3 = 10 ppm
PO4 = 0.6 ppm
Fe = 0.05 ppm
The only thing remaining is to get higher my macros.