What can influence a tank's Mg reading

[Delphinus]

So, just updating this thread with additional observations:

1) (And just to echo what's already been said in a couple other threads lately) I can't beleive how much Mg it takes to raise Mg in the tank. I ordered 20 lbs from Chemmaster and I've used up 4 lbs in the last TWO WEEKS. (Over 3 tanks and in new water change water mind you.)

2) I've used up my Elos test kit and am now finishing up my Salifert. One thing I am finding with the Salifert .. and I'm not sure if this is an artifact of: 1) the age of the test kit (about 1 year or so), or 2) the Salifert Mg test kit in general, or 3) normal day to day fluctuations of Mg in solution,or 4) maybe I suck at using test kits ... but I can't beleive how inconsistently my tanks' Mg levels are from day to day. For example, I was targetting 1400 and I was adding Mg at certain amounts and testing each day. So say I wanted to raise Mg by 50 per day, I would try to find the amount that would do this. But the same amount of additive would never raise the levels by the same amount. And off by factors of 3 or 4 in some cases. Ie., one day, 15ml would raise a tank 15, but another day, 15ml would raise a tank by 45..or 135. HUH?? ... Also noted that the amount from tank to tank was not directly proportional to water volume (ie., my 40g took an amount that was not anywhere near twice that what was for the 20g, and the 115g took even more proportionally. It's like it's by "water volume squared" maybe?) Weird??? Even weirder, my 110g was the first to hit 1400. So I stopped adding Mg because now I'm shifting from "raise Mg" into "maintain Mg" (and for that I want to determine what the consumption rate is on, say, a weekly basis, then I'd find out how much it takes to replenish that amount, divide by 7 and hey presto, there's the amount to dose each day). But the next day, without adding any Mg, the Mg level had jumped 30 on its own.

Me confoozled by this.