Even with frequent water changes your trace levels can still be low. I do 15-20% water changes every 2 weeks and my magnesium levels would barely get above 1100, so I thought, wtf is going on... so I decided to test the water I was buying from my LFS (they use Instant Ocean, not Reef Crystals) and the magnesium level using a salifert test kit was at 1050! Now it all makes sense. So I have decided to start buying and mixing my own water so I dont have to constantly dose magnesium to keep my levels up. I figure this will save me money in the long run too.