Magnesium - Calcium - Ph - Alkalinity


Ok this is the situation
My system consists of
- temporary DT of 151Gl Total water volume
- Sump of 187Gl total water volume and about 150Lbs of life rock
- Fuge/ Seahorse display of 105Gl
which makes the complete system a 443Gl system.
I use fresh Ocean water which comes out of the gulf of Thailand which is checked by myself and has all parameters sitting at ideal sea water levels. (as per most websites stated as ideal sea water levels that is.)
now the problem.
Due to some missed water changes I faced some Nitrate/Phosphate issues. Low issues though but issues nonetheless. Anyway those have been dealt with and are recovering slowly due to new water changes and some phosphate lowering treatments.
Anyway as my levels of Magnesium and Calcium were very low due to the missed water changes they still havent recovered since?
They currently sit at
Ca 420
DkH 6.1
Alk Meq/L 2.16
Mg 1170
my Ph swings from 7.92 at night to 8.09 midday
Salinity 1.025
Temp 77.3 - 78.6
How to deal with this?

- I was thinking dosing baking Soda to raise my Ph which would bring my Alkalinity back to normal?
- dose magnesium?
- Just continue with large water changes and sit it out?
any advice would be highly appreciated here


Well-Known Member
Your parameters are so close you really don't have a problem. My cal kept dropping to 380 and PH at night drops. I started dripping Kalkwesser, it's really easy and it seems to be stablizing things for me. You drip the kalk real slow through an IV line at night to make up for evaporated water that you normally top off.
Kents doser is used on my tanks right now, but I also have a digital doser pump that I just haven't put to use yet. The digital doser lets you program how many drops per hour. The Kents doser just sits above the water line thats in the sump and a little roller controlls the flow. I put 5 teaspoons of kalk per 5g of water...give it a quick stir to mix and let it sit about 2 hours to settle before use. You only use the middle water, the 2 inches to the very bottom has white sludge and the top thin layer is crusty.


Well-Known Member
Alkalinity seems tobea little low. Baking soda will bring alkalinity up but pH down. You should bake the baking soda on a cookie sheet for one hour at 400f to drive off the carbon dioxide in the baking soda. Using baked baking soda will bring your pH up and stabilize your alkalinity. It could just be that your tank is using that much calcium and alkalinity to maintain coral growth.
Regular calcium chloride does a good job adding calcium to the tank. 250g of calcium cloride dissolved in one gallon of distiller water is a good supplement to use. You can find instructions for all of this if you do a search for randy holmes-Farley recipes. They will also tell you how much to dose.
For magnesium, Epsom salt is a good source. Some people are able to find cheap magnesium cloride as well. You will find that recipe too on there.
Also, double check your test kits for accuracy.


Active Member
I would do some bigger water changes to see if you can make head way on it first. If you have any of that reef salt mix use it instead of Ocean water. If that does not work for you then I would start to learn how to adjust the water through additives.


now this morning I found out that my Ph actually drops to 7.80 throughout the night.
Is a swing from 8.05 to 7.80 normal?
Would that have to do with my other measurements especially the low Alkalinity?
I also had my skimmer logged off last night due to some maintenance issues could this have contributed in my low Ph this morning?

bang guy

That's a perfectly normal PH swing.
Since you have both Magnesium and Alkalinity low I really recommend a very large water change. Pretty much as large a change as you can handle.
The issue is that Alkalinity will continue to drop unless you can get Magnesium higher. Epsome Salt is fine for a 20 - 30ppm increase in Magnesium, perhapes even up to 50ppm, but there is so little Magnesium in Epsome Salt that you would need a massive amount to raise Magnesium by 100's of ppm and that's an aweful lot of Sulphur to add. Magnesium chloride would be more efficient but at that point you really are better off just doing the water change(s).
I really would ignore your PH right now until you get your major elements balanced.


I will be doing a 110Gallon (as much as I can store) water change tomorrow then and maybe schedule another one 4 days later.
see what that would do.