Ca and Mag supplementation for fresh mixed water


New member
Just opened a new RC bucket and I'm getting the following for 24 hours fresh mixed water:

Sal: 1.025
Ph: 8.37
Temp: 80
Alk 12.2
Ca 370
Mg 1095

Current tank parameters:
Sal 1.025
PH 8.2-8.4
alk 8.3
Ca 380 (bringing it up to 420 +)
Mg 1140 (bringing it up to 1300)

Do you think I should supplemnt the fresh mixed water to avoid bringing my parameters down and messing up my 2 part daily dosage? Should I supplement in the salt mixing bucket or in the tank after the water change reason for this question:
- will added Ca precipitate if added in mixing bucket where alk is 12 .2?
-If I perform the water change and then supplement wouldn't that cause to much fluctuations for the fish to handle?

Last question, is about alk would a 30G water change with water at 12.2 shock a 150G tank where alk is at 8? I used to use 50% of Crustal sea marine mix with an alk of 9 to lower it down but it has very low ca (300) and Mg (960) and ended up to costly to supplement.
Thanks for your help.


New member
Its a bit less than 30 G.
Will retest with a different testkit tomorrow as I tested it with Salifert which I as I've read isn't the best testkit for Ca?
Any info if supplementing pre water change will cause some precipitation?

Randy Holmes-Farley

Reef Chemist
Premium Member
No, boosting calcium and magnesium to normal levels will not cause precipitation at 12 dKH, as long as you do not drive up the pH somehow. :)