What uses Mg?

[kevensquint]

Hi I dose 2 part and limewater ( night) to my sps reef. For some reason it's been 3-4months since I've needed to add Mg. Yet my tank uses 30 ml 2 part per day and 200ml limewater.
I have also a mangrove/chaeto 40 gallon refugium plumbed to the 160 sps display. I have in fact found K to be used up quite a bit and needed to dose 125ml last week to get it to 400ppm. I am lost why my Mg is always 1400-1500ppm and I never add any, nor does it seemed to be used. I do 20% WC per two weeks with aquaforest reef or RSCP.

 

[chipmunkofdoom2]

Randy Holmes-Farley wrote a great article on Magnesium. This table lists what organisms use magnesium. Another article of Randy's on Kalkwasser also shows that small amounts of magnesium also present in calcium hydroxide. Bulk Reef Supply's site lists RSCP as having 1,280ppm - 1,340ppm magnesium at 33ppt salinity. If you scale that up to 35ppt, that's between 1357ppm and 1421ppm. Aquaforest's salts have fairly high magnesium as well. BRS lists the magnesium between 1,300ppm and 1,380ppm. It is unclear if this is at a salinity of 33ppt or 35ppt. If this is indeed at 33ppt, then adjusted to 35ppt, that is 1,378ppm to 1,463ppm.

My guess is that your calcifying organisms are using magnesium, but because the usage is so low, and because you're replensing magnesium with water changes and kalkwasser, your magnesium levels remain high and constant.

 

[Randy Holmes-Farley Verified Community Expert]

How are you measuring magnesium?

Some salt mixes just start with such high levels that it doesn't get used up even down to NSW levels.

 

[kevensquint]

I use the red sea test for Mg , Alk, Ca. I use hanna for PO4 and salifert for NO3 and K. I use these same kits to test about 10 other local systems and the results do vary from tank to tank. So I have estimated the test kit is probably ok. It has recently read 1150 ppm of Mg on another tank.