What can influence a tank's Mg reading

[Delphinus]

I fell off the wagon for Mg testing, when I got back on it recently I found my tanks vary from 900 to 1100. So, been adding Mg to each tank at the same proportions. Two out of three tanks have been showing the same increase per day, as you would expect. However the third is not showing any increase. I've even doubled up the Mg dosing on that tank for a few days now and it remains steady at 900. Were it not for the other two tanks showing increases I would suspect the test kit or the supplement, but we can safely rule those possibilities out.

Is 900 maybe a sort of low water mark for the Elos Mg test kit? Ie., could it be that it's sub-900 but the test kit is unable to show a reading of less than 900?

Trying to think of other possibilities such as perhaps the Mg is simply being absorbed into the biology of the tank but it seems strange to me that the levels are flatlined instead of showing marginal increases or decreases. The resolution per drop on the test kit is 75 so it could be that it is increasing at very small increments and simply hasn't surpassed the threshold to read the next level (975) but again given that the other two tanks are increasing at roughly 75 per day and this tank's proportional additions are now twice that level, it just seems odd to me.