Sodium Carbonate Alk Swings!

R6REEFER · Jan 24, 2016

So I have a 160 gallon system that naturally has super low ph due to the apartment being air tight. I started dosing sodium carbonate about 2 months ago and finally got it dialed in to where I have a constant ph of 8.0. It is dosed via the ATO at the moment. I've been keeping a close eye on mag and alk. Mag because I use Kent marine salt which yields somewhat lower mag levels than my favorite, instant ocean. I've been testing every 2 days for the past week. I was getting a solid 11.2 dkh every day until today. I did a 10% water change and now my alk is at 9.5dkh. I'd way rather have 9.5 over 11.2 but my goal is repetition. I understand the WC can swing alk a bit but not that much! What exactly can be causing this problem and how can I fix? My other parameters are great aside from a tad bit low mag and of course the alk swings. 2 weeks ago I had another swing from 9.7 to 10.8. Any recommendations? If I don't dose
Na2CO3 I will have a ph value of around 7.2. I can't run an air line from outside to skimmer or pump, that's out of the question.

Randy Holmes-Farley · Jan 25, 2016

When doing a water change, the resulting alkalinity will be the weighted average of the alkalinity in the new and old water.

So a 25% change with 7 dKH water on a tank at 11 dKH will produce a tank with 0.25 x(7 dKH) + 0.75 x(11 dKH) = 10 dKH

If you saw a bigger change than that equation predicts for your situation, something is amiss, either one of the measurements of alk or the estimate of water percentages (or alk changes for some other reason, such as altered dosing).

Personally, I would not aim for a particular pH, but use high pH additives to attain your desired alkalinity, then either live with the pH, or look to other options (such as a CO2 scrubber on the skimmer inlet). Limewater (kalkwasser) will give about twice the pH boost per unit of alkalinity as does sodium carbonate. It would be good to keep the pH minimum above about pH 7.8.

R6REEFER · Jan 25, 2016

Thank you very much! The equation does not add up on my side so it's got to be something else. I'm using 18.8 grams Na2CO3 per 5 gallon ATO fill. I believe I will take your advise of trying to dial in the alk rather than the ph and see what happens. Alk isn't going to be a life or death situation in my tank as its going to be primarily chalices, BUT practice makes perfect. All of my tests are done twice, my red sea tests can get pretty squirrley. I use 3 different tests for alk (RS, seachem, and API) and a Milwaukee ph meter for ph as well as an API test to back it up. Do you think a calcium reactor would help solve? I'm kind of on a budget as I just installed 2 MP40QD'S and 3 hydra's this weekend :s

Randy Holmes-Farley · Jan 25, 2016

R6REEFER said:
Thank you very much! The equation does not add up on my side so it's got to be something else. I'm using 18.8 grams Na2CO3 per 5 gallon ATO fill. I believe I will take your advise of trying to dial in the alk rather than the ph and see what happens. Alk isn't going to be a life or death situation in my tank as its going to be primarily chalices, BUT practice makes perfect. All of my tests are done twice, my red sea tests can get pretty squirrley. I use 3 different tests for alk (RS, seachem, and API) and a Milwaukee ph meter for ph as well as an API test to back it up. Do you think a calcium reactor would help solve? I'm kind of on a budget as I just installed 2 MP40QD'S and 3 hydra's this weekend :s

A CaCO3/CO2 reactor would make the problem MUCH worse as it lower pH while the sodium carbonate raises it.

R6REEFER · Jan 25, 2016

Okay, I'm going to go ahead and delve into limewater. I'll use you're two part solution

R6REEFER · Jan 25, 2016

Randy Holmes-Farley said:
A CaCO3/CO2 reactor would make the problem MUCH worse as it lower pH while the sodium carbonate raises it.

I have 50lbs of Sodium Carbonate, can I make my own limewater?

AllSignsPointToFish · Jan 25, 2016

I believe limewater is calcium hydroxide dissolved in water, not calcium carbonate.

Randy, how much calcium hydroxide do you typically add per gallon? (This is a question I've been meaning to ask for quite some time)

R6REEFER · Jan 25, 2016

Good question, I'm looking for a good supply of calcium hydroxide atm. I would like to know a good starting point as I don't want to bomb the tank again. I way overshot the sodium carbonate my first time which caused a brutal ph swing. Pretty much all my sps died, luckily it wasn't much.

R6REEFER · Jan 25, 2016

AllSignsPointToFIsh said:
I believe limewater is calcium hydroxide dissolved in water, not calcium carbonate.

Randy, how much calcium hydroxide do you typically add per gallon? (This is a question I've been meaning to ask for quite some time)

No, I was way off. You can start with CALCIUM carbonate, bake it in oven, and add water to hydrate which would have an end result of calcium hydroxide. The correct time in oven or amount of water is unclear to me. Sodium carbonate is something totally different though it has similar results in the water column.

AllSignsPointToFish · Jan 25, 2016

Technically, the calcium carbonate decomposes to calcium oxide when heated. The calcium oxide then has to be dissolved in water so as to prevent the oxide from reverting back to the carbonate by reacting with carbon dioxide in the air.

Calcium oxide is unstable and reactive, so use care when handling the stuff. It will probably react with moisture in your skin, causing chemical and possibly thermal burns.

AllSignsPointToFish · Jan 25, 2016

Also, the temperatures required for converting the carbonate to the oxide are quite high and probably not achievable with most kitchen appliances.

biom · Jan 25, 2016

Yea, calcination (CaCO3=>CaO+CO2) will start above 825 °C (1,517 °F), I don't believe if there is kitchen oven that could reach even half of temperature needed.

AllSignsPointToFish · Jan 25, 2016

Maybe a toaster oven

Randy Holmes-Farley · Jan 25, 2016

R6REEFER said:
I have 50lbs of Sodium Carbonate, can I make my own limewater?

No. Limewater is made from calcium hydroxide.

Randy Holmes-Farley · Jan 25, 2016

AllSignsPointToFIsh said:
I believe limewater is calcium hydroxide dissolved in water, not calcium carbonate.

Randy, how much calcium hydroxide do you typically add per gallon? (This is a question I've been meaning to ask for quite some time)

I get mine close to saturation, usually 80-90% saturated, which is about 2 teaspoons per gallon, but I d it by conductivity of the product water, not measuring the teaspoons.

AllSignsPointToFish · Jan 25, 2016

Cool. Thanks!!!

R6REEFER · Jan 25, 2016

biom said:
Yea, calcination (CaCO3=>CaO+CO2) will start above 825 °C (1,517 °F), I don't believe if there is kitchen oven that could reach even half of temperature needed.

HAHA no, I don't believe so. It didn't specify how hot, it just said heating and hydrating!

beaslbob · Jan 25, 2016

R6REEFER said:
Good question, I'm looking for a good supply of calcium hydroxide atm. I would like to know a good starting point as I don't want to bomb the tank again. I way overshot the sodium carbonate my first time which caused a brutal ph swing. Pretty much all my sps died, luckily it wasn't much.

I believe pickling lime is calcium hydroxide.