Alk. Issue please help

Randy D

So I am having an Alk. issueI can't seem to wrap my head around. I tought I had a basic understanding of reef chemistry but apparently not so much.
Over the last 4 to six months I cant' keep my Alk. above 7ish. I used to keep it around 9ish with 1/2 as much dosing.
I am stumped as to why.

180 gal total volume, 120(4') display, 30 gal fuge low flow with algae and rocks (in garage), sump in basement
I used Red Sea Coral Pro salt for at least 2 years now.
I use 2 dry cuprs GFO and a RO cart. of carbon which I change out every month
I use saturated limewater for top off (about 3 gal. evap per day), have been using this since day one due to low PH issues.
I use BRS bulk products for my Alk., Ca, and MG dosing

Current test results ( I use Hanna and Salifert to check each other)
Alk. 7.3
Ca. 440
Mg 1300 (Itry to keep this about 1400)

I m dosing 450ml of ALK and 400ml of Ca. (I have checked calibration of doser to be sure of these numbers.

So to conclude I can't figure out why I can't get my Alk. to the old range of 9ish. something has to be causing this 2 point drop in Alk.

I have attached a not so great pick of my display The algae in my tank that I have an issue with is the red mat like stuff on my rocks and some on the bottom. Very hard to find the proper way to remove it.

Comments and suggestions are needed and welcomed.



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Staff member
RC Mod
Are you sure of your mg reading? See if you can get a confirmationi from your lfs on that. Test kits go bad over time, and if you have a bad alk reading it could sink the alk; otoh, it could be a bad alk test kit.


Moving on Up
I know crazy right I used to dose about 250ml per day to get to 9 ish. This is why I am stumped

Have you tested freshly made saltwater with your test kit? Something isn't right. I does 40ml a day in a 260 granted I don't have a heavily stocked SPS tank but that's a lot of dosing.

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Randy D

I checked my salt mix for ALK when I did a WC Wed. the ALK on the mix was 11.9 right where it should be.

Good thought on the Mg test. I will check it as the salifert kit I have is a little old.

I understand on the dosing amount it really confuses me why I have to dose so much product.


Premium Member
I would try dosing by hand for a few days, in addition to getting second opinions on the test kit. Are you seeing any signs of precipitation, i.e. whitish buildups, on tanks walls or maybe in the GFO or other media?

When you tested the calibration of the dosers, did you measure the actual flow rate into the tank?

Randy D

Ok here is an update. Have good numbers on the Mg and it is now 1450.

Real time numbers today.
FYI I am dosing BRS washing soda.
I have stopped auto dosing Alk.
yesterday Alk reading 7.0 manually added 8oz or 237ml of BRS
Today 24hrs after above dose Alk reading 7.0
added 8oz or 237ml and checked 2 hrs later Alk reading 7.3

I just don't understand why I am having an issue raising Alk. My PH spikes through the roof after adding BRS but in a couple of hrs settles down to 8 to 8.1.

Any Ideas


New member
It's precipitating out …..try using bicarbonate (baking soda). your ph spiking is causing the precipitation...


Premium Member
I would try dosing baking soda by hand for a while. The lower pH will tend to reduce any precipitation. I'd also dose into the largest volume of water, generally, the display tank. You can measure the alkalinity 10 minutes or so after dosing. Waiting very long might show consumption effects.

The pH drops fairly rapidly after dosing due to aeration bringing carbon dioxide into the water column. pH buffers increase the pH by "consuming" carbon dioxide, loosely speaking.

Randy D

Well I think you all are on to something. Mixed up some baking soda solution and 1 hr later 8.8 Alk. PH did not spike went to 7.9 actually dropped a little. I will continue this method and see what happens.


Premium Member
Baking soda should drop the pH slightly when dosed. As you've seen, the effect is temporary as aeration will dump carbon dioxide from the baking soda back into the air.

Randy D

Yes I noticed the PH. I did a WC today and stopped all dosing. going to check parameters and readjust my dosing using baking soda instead of washing soda. I would think this change would change the amount of CaCl that I have been dosing for Ca.

BTW does anyone have an opinion on the continuation of limewater as my evap. replacement? I have always done this as I said for the low PH issues and high CO2 in my house.


Premium Member
Limewater should be fine to use as long as the pH level is acceptable. It might be interacting badly with the washing soda, but that's not clear at all.