It was a test kit giving very low readings 3 times in a row.
If in doubt with your odd readings, go get another test kit before you mess with your tank.
I added a lot of magnesium, just to figure out later I was misled by the test kit.
By the way, the annoying main tank algae is turning gray in some pathces either from the Magnesium overdosing or my huge ball of Caulerpa in the sump.
I just hope it dies off slowly.