PH higher in fish tank than make up water?

[Cecil]

Ladies and Gentlemen,


I finally was able to break down and buy a PH meter. Unfortunately there have been other priorities. I was dying to know and monitor the PH in my RAS water, but other things came first as long as the fish were growing and healthy.

Anyway, my RAS fish tank water is 8.1 PH while the make up water is 7.5 right out of the well. I would have thought it would be the other way around. Don't the autotrophs reduce the PH? Why would it go up instead of down?

There is some hydrogen sulfide blown off initially when I fill the make up water tank/iron filter. And apparently there is definitely some C02 in the well water as I can watch my C02 meter on the wall rise as I fill the make up water tank. Perhaps the PH rises after the elimination of those two compounds?

I thought I had this all figured out. I guess I didn't. LOL

[Ravnis]

Co2 dissolved in water forms carbonic acid. The water was probably under pressure in the well which caused it to absorb more CO2. Once it came to atmospheric pressure it most likely gassed off and raised your ph. Just a wild guess.


Does this mean my water chemistry won't make it necessary to add lime as some operators have to?

[Crusty]

Your alkalinity would tell us more about the CO2. What is your pH and alkalinity of the source water after treatment prior to adding to the culture tank?

[Cecil]

Your alkalinity would tell us more about the CO2. What is your pH and alkalinity of the source water after treatment prior to adding to the culture tank?

Total Alkalinity as CaCO3 343 mg/l. Source water 7.5 PH right out of the ground with no treatment.

[Crusty]

Then after that, what are the readings after treatment?

[Cecil]

Then after that, what are the readings after treatment?

Don't know. The well water was tested by a lab. I don't have the means to test those parameters at home.

Probably higher due to those ions not being evaporated which causes the PH to climb? As one poster on the NANFA site stated, I may be adding more and more carbonates with my make up water?

[Crusty]

Don't know. The well water was tested by a lab. I don't have the means to test those parameters at home.

Probably higher due to those ions not being evaporated which causes the PH to climb? As one poster on the NANFA site stated, I may be adding more and more carbonates with my make up water?
True about the carbonates, which Ravnis posted. But the CO2 levels are relative to your ph and alkalinity (with some measure of temperature). At the levels coming out of the well your CO2 levels are (off the top of my head) around 25ppm which is not a big concern. The addition of that water into a lower pH will change the CO2 significantly because of the lower pH but still not much of a concern. This is purely aquaculture Cecil?

[Cecil]

Yes, purely aquacultural. I want to get into aquaponics but would like to get the fish aspect down pat, and do some experimenting with different biofilters and clarifiers to find the best combination before I get into plants.

I'm not concerned about the PH rise, just surprised as it seems counter to all the literature I have read. Here's an email response from Dr. Gary Miller down the road that raises hybrid striped bass in an RAS for the gourmet market in Chicago. Thoughts?

Cecil;

Collect some well water directly into a jar -- not full -- , cap it and
agitate
like crazy. Now collect another sample of well water and compare the pH
levels.
My guess is that the well water (after aeration) has a pH closer to 8.7
because
of the high alkalinity we have around here. If your sample isn't that high;
areate it further.

[Cecil]

Mystery solved. (At least it was a mystery for me).

I did the jar test as directed and over time with aeration (agitation in this case), PH indeed does climb. I has climbed to 8.1 in the jar and according to Dr. Gary Miller will climb to 8.7 at peak. We shall see.

So apparently liberation of C02 does raise the PH in my ground water. If the final PH is indeed 8.7 that means the bacteria on the biofilter may be reducing the PH after all, which is to be expected.

I was never concerned about this, just puzzled. I would prefer it to be a little lower to keep the percentage of unionized ammonia down, but I suppose as long as I keep the ammonia at zero I should be O.K.

[Ravnis]

I wonder how much is co2 off gassing versus o2 absorbing. Oxygen in the water raises ph. Either way you should get some alkalinity added every top off.

[Cecil]

I wonder how much is co2 off gassing versus o2 absorbing. Oxygen in the water raises ph. Either way you should get some alkalinity added every top off.

Don't know but with my packed column I used with the borehole water entering my trout pond I was able to get saturation of oxygen up to about 90 percent.

It would be interesting to see what the alkalinity is. I may get a test kit down the road sometime.

[Crusty]

Hi Cecil, the kits are about 10 bucks mate and will do you for a few years.

Predicting the degassing of co2 is a little bit difficult but we can work out the difference in saturation from the alkalinity and pH with some rough results. If we assume the same alkalinity in both the source water 343mg/L and adjust for the pH change we end up going from source pH of 7.5 and 21.64mg/L CO2 to tank 8.1 and 5.43mg/L CO2 that is a change of 16.21mg/L . I doubt that means much because we have not taken into account the respiration of the fish. But if your alkalinity in the culture tank read the same, that would be close to the CO2 saturation. Fun to work out, because I like math.

[Cecil]

Thanks Crusty!