Mystery of decreasing pH after water changing with higher pH water? - The Planted Tank Forum
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post #1 of 7 (permalink) Old 10-01-2020, 03:53 PM Thread Starter
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Mystery of decreasing pH after water changing with higher pH water?

I'm in the 4th week of a new 50G. It is 80 percent planted at this point and most everything has doubled in size. There are maybe 30 endlers of various size, from fry up. And there is an abundance of shrimp. It's obviously still cycling although it has been days since I detected any ammonia. I have not yet detected nitrites.

My nominal pH, the pH when CO2 is not being injected, has dropped from a high of 7.6 and seemingly each time I've done a water change. I just did a 50% change last night and nominal pH dropped from 7.2 to 7.0. But here is the rub, my tap water is pH 7.6, so why would this be happening? Here are the water parameters:

Out of the tap: pH 7.6, 5dKH, 11dGH
Tank prior to 50% water change: pH 7.2, 5dKH, 14dGH
Tank after 50% water change: pH 7.0, 5dKH, 12dGH

I have an abundance of Seiyru rock which will creep my KH and GH up over time and seems to stabilize around 7dKH and 16dGH if left without water changes. I would assume this rock would RAISE my pH however. I also have one largish piece of Manzanita. It was sandblasted so it is not red and has not generated any tannins that I have noticed.

I have been targeting a pH drop to 6.5 for CO2 enrichment. One theory is that the CO2 is not completely out gassing during time it is turned off and this is causing my pH to appear lower even when CO2 is off for hours. I have reduced current in tank recently because it was blowing fish and shrimp all over the place, so maybe? But I run an airstone every minute that CO2 is not on so not sure this theory holds up. And I have two 200GPH canisters even though I added foam to blunt the current.

Another theory is that this is part of the cycling, in particular when nitrites are produced as those are apparently acidic. But I have never detected nitrites. Ammonia was detected at <.25ppm several times although ammonia has been absent for a few days now. Nitrates range near 0ppm to 5ppm (I have been dosing with Seachem Nitrogen when nitrates are low). Besides water changes, I have managed ammonia with full tank dose of Seachem Prime 2x/day, Seachem Stability and Pristine daily.

A third theory is that it may be due to dosing with Seachem Potassium, which I just learned Potash may be an acidic source. I do notice a temporary pH drop when adding to the tank as I pour it in the tank near the Milwaukee pH controller probe. But that pH drop seems temporary and it comes back up. I have been dosing K at the ~ 2ppm / day level (the dose on the bottle instructions).

Not sure this has anything to do with it, I am using Seachem Flourish tabs and due to that and fish load, I dose lean except for Fe and K. Substrate is inert Seachem Flourite Black Sand (with Flourish tabs).

Any ideas on what can cause this mystery pH drop?
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post #2 of 7 (permalink) Old 10-03-2020, 05:36 PM
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My best guess would be the water change water not completely degassing after it comes out of the tap. I like to keep my tap water change water in the buckets for a few days just to let chlorine gas out and ph drop.
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post #3 of 7 (permalink) Old 10-05-2020, 04:04 AM
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Water parameters can be totally different from out of the tap and letting it age for at least a day. What causes your parameters to change the day after your water is the degassing. Aging water is good for two reasons: getting rid of the chlorine and letting the water degass. I always having water aging around for either topping off my tank or prepping it for a water change. Good luck with your fish tank.
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post #4 of 7 (permalink) Old 10-09-2020, 04:52 AM Thread Starter
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What degasses from tap water that will cause a pH drop over time? This might be the issue since I am taking water straight from the tap. The process of pouring it in the tank might degass it, thus resulting in the lower pH as soon as I pour the higher pH tap water into the tank.
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post #5 of 7 (permalink) Old 10-09-2020, 01:08 PM
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I suspect that the bulk of the issue is your suggestion that CO2 is not fully degassing between cycles. It can easily take longer than overnight to fully degas. Gas exchange issues can account for a lot of this. You can test this by letting some tank water sit out for several days and comparing the pH to expected pH levels.

The cycling aspect may also be contributing. If you think that your tank is still throwing off a lot of NH4, then your BB haven’t developed to the point where NO3 is being created. Does your dosing account for all of the NO3 your test kits show? Then, if plants consume a lot of NH4, vs NO3, the plants will release H+, which reduces pH.
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Last edited by Deanna; 10-09-2020 at 01:19 PM. Reason: add
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post #6 of 7 (permalink) Old 10-09-2020, 05:41 PM Thread Starter
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Quote:
Originally Posted by Deanna View Post
I suspect that the bulk of the issue is your suggestion that CO2 is not fully degassing between cycles. It can easily take longer than overnight to fully degas. Gas exchange issues can account for a lot of this. You can test this by letting some tank water sit out for several days and comparing the pH to expected pH levels.

The cycling aspect may also be contributing. If you think that your tank is still throwing off a lot of NH4, then your BB havenít developed to the point where NO3 is being created. Does your dosing account for all of the NO3 your test kits show? Then, if plants consume a lot of NH4, vs NO3, the plants will release H+, which reduces pH.
The more I think about it, CO2 degassing causes a rise in pH so it can't be that. Jostling the water around by pouring in new water should expel some CO2 from the motion and cause pH to rise.

I doubt its NH4. I test ammonia daily (API drop test) and have not had any ammonia in a couple weeks now. My nitrates usually measure 0ppm each morning just prior to dosing. The tank is now in 5th week, had ~ 20 endlers and ~ 100 shrimp for a few weeks, Seachem Stability and Pristine (bacteria in a bottle) dosed daily from day 1. I have two canister filters chock full of different types of biomedia. I'm guessing I am most of the way to cycled although I know the bacteria colony gets stronger and more resilient over a longer period of time. But I think there is enough there to handle the nitrogen cycle.

I'll try a test and let some tap water sit for a while. It's close to 7.6 pH out of the tap. So maybe the mystery solution is that it's not really 7.6 water, if it sits out or gets disturbed, it becomes 7.2 water. But what would outgas from tap water that would cause its pH to go lower? Does dissolved chlorine gas create an acid? And I usually put Prime in the pre-tank bucket so it mixes with the water while filling, thought that might neutralize chlorine but I don't know anything about the chemistry of that process.
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post #7 of 7 (permalink) Old 10-09-2020, 06:50 PM
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Quote:
Originally Posted by ahem View Post
The more I think about it, CO2 degassing causes a rise in pH so it can't be that. Jostling the water around by pouring in new water should expel some CO2 from the motion and cause pH to rise.
That is exactly the point: if the CO2 isn't completely expelled, the pH won't rise to the level of totally degassed water. Tap water often has quite a bit of CO2 in it, so that may not be what you think. Your test with degassing a sample of tank water should give you some better insight into the CO2 issue.


Quote:
Originally Posted by ahem View Post
I doubt its NH4. I test ammonia daily (API drop test) and have not had any ammonia in a couple weeks now. My nitrates usually measure 0ppm each morning just prior to dosing. The tank is now in 5th week, had ~ 20 endlers and ~ 100 shrimp for a few weeks, Seachem Stability and Pristine (bacteria in a bottle) dosed daily from day 1. I have two canister filters chock full of different types of biomedia. I'm guessing I am most of the way to cycled although I know the bacteria colony gets stronger and more resilient over a longer period of time. But I think there is enough there to handle the nitrogen cycle.
If you get an ammonia reading, that means that your plants and BB aren't able to keep up with it. The organic stream is constantly creating ammonia. Your suspicion that, perhaps, you weren't fully cycled (no NO2 or NO3) would mean that there are no / or insufficient BB to handle the ammonia and, if you aren't reading ammonia, that only leaves plants to consume it. At your pH levels, the ammonia is going to mostly be in the form of ammonium. As plants consume ammonium, pH can drop.
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co2 enrichment, ph drop, seiyru stones, tank cycling

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