Magnesium levels?

[Douglas]

Can/do Magnesium levels fluctuate during the day? Without dosing Mag in three days, my level has fluctuated from 1350 at 5:00am yesterday to 1550 at 7:00pm tonight. I tested it twice, because i couldn't believe the reading myself. I have been trying to determine how much Mag is either consumed, or used in my tank, so i can place magnesium on my doser. But with inconsistant readings, it's becoming hard to determine this amount. Any ideas would help. Thanks.