Why does my MG keep droping?


New member
My 40g reef has had a problem keeping magnesium levels up, they tend to drop about 150ppm per week. About two months ago I had low calcium so I got a magnesium test kit, found out that that was probably the problem. It was about 1170 on 5/11 raised it to about 13-1350 then was back at 1170 on 5/21, raised it agian. I moved my tank about a month ago, 50% water change was at 1250 or so after but I had no magnesium sulphate so it stayed there for 2 weeks along with 380 Ca. About a week and a half ago I raised it to 1500 by accident, should have done a half way check but no ill efects, and now its back to 1350, right where I want it. And Ca has been at a steady 450ish since I raised the Mg to 1500.

So Im just curious does magnesium really deplete that fast? I have several frags and one 2-3" colony of sps that grow semi slow but have good coloration, and 3 small colonies and a few frags of acans.

The rest of my parameters are pretty good as of a few min ago with API test kit:
Kh-7.5 (just added b ionic to raise .5 DKh going to get it up to 9-10)
PO4- 0 but really who knows?

I have zero algae so I think the phosphate reactor is keeping that at 0. I'm planing on getting new test kits but with no algae, feeding 2 times a day and everything looking pretty good and some growth I dont think i have a nutrient problem at all. Also my Mg test is salifert so I know its reliable.

Im really just curious if Mg use is that high, I dose about 7ml of b ionic a day, probably need to dose more, and if my Mg keeps dropping like this then Ill need to add about 3-4 oz a week of magnesium sulphate which I just think is a lot

Thanks for any input


New member
So Im just curious does magnesium really deplete that fast?

yes :)

i use mag chloride and mag sulphate dissolved (5 cups and 2 cups respectively, i think!) in a gallon of ro/di...so while i dose my ca and alk, every friday we pull water for testing then i pour in a quart to half gallon. i then adjust the next week's mg addition, and start the whole thing over when i run out of my gallons of ca and alk.

yeah...for me to get caught up to start with mag i used a ton. A TON. lol good luck!



New member
Generally Mg is depleated pretty slowly, but there are some things that can effect the useage rate Mangrove and other green plants come to mind as larger consumers but your system seems to use them excessively fast.

Maybe get a second oppinion on the test results with another good brand of kits.


Premium Member
I agree with jw. Magnesium normally depletes rather slowly.

One thing to keep in mind is that out of all the hobby grade test kits we use, the mag test has the most test noise.

Meaning, you must be really, extra, extra careful when performing mag tests. They are quite touchy.

I suspect your mag level is fine. :)

Randy Holmes-Farley

Reef Chemist
Premium Member
I agree. Unless you do water changes with a very low magnesium mix, I do not believe that a normal reef tank can possibly loose 150 ppm of magnesium in a week. It is probably testing error.


New member
Ok I'll re test my water in a few days after I have gotten my KH up 2 degrees or so and do it really carefully. I think it is also good because I can maintain my calcium levels a lot higher than I was able to before. My sps have grown about 1/8 of an inch in the month since I moved my tank so I think that my parameters are ok. Im just hoping to get them great so that my growth can get a bit better to 1/4 inch a month.