Odd pH Shift, Why?

[Unexpected]

I run 0dKH and have pool filter sand. I believe the sand is not completely inert as I can see my pH trend up throughout the week. This is a very small trend and easy to manage, but as of yesterday and today, my overnight pH is much lower than expected and I have had to increase CO2 to compensate. I've seen this before and chalked it up to a pH probe issues but I'm now doubting my logic. What would cause this? I grow plants hydroponically and tested pH with my liquid kit and have verified the pH is accurate with the probe.

Any thoughts? This occurred after my last water change.

 

[Unexpected]

Wait, please tell me I'm crazy and weather changes don't effect my tank. I haven't run the AC in 2 days and the house has been opened these last 2 days.

 

[Freshflora]

Wait, please tell me I'm crazy and weather changes don't effect my tank. I haven't run the AC in 2 days and the house has been opened these last 2 days.
View attachment 3054

Is the temp in your tank higher? That’s the first thing that comes to mind—higher temp so less O2 and thus less offgassing overnight?

 

[Unexpected]

Is the temp in your tank higher? That’s the first thing that comes to mind—higher temp so less O2 and thus less offgassing overnight?

The spikes up are water change days as my reservoir is in the garage with higher temperatures.

 

[RickyV]

Maybe try @Yugang method for checking what the degassed pH is in 10 minutes. Perhaps it is still at the same point it was before. To me it looks like less off gassing is happening overnight, I'm guessing no change in the actual amount of CO2 injected. Maybe filter is clogged and you have reduced surface agitation? This one seems unlikely especially with the house being open, but maybe elevated CO2 in the house due to the A/C being off.

 

[Count Krunk]

If youve had your windows open for two days. That would drastically lower the co2 level of your house. It could totally affect your ph.

Reefers often run lines from their skimmer to the outside to push up the ph in their system.

 

[Yugang]

checking what the degassed pH

I would also vote for this, both for tank water and WC water. What you observe may or may not be CO2 related, and you may want to rule out some other substance that affects pH, or perhaps KH.

my overnight pH is much lower than expected and I have had to increase CO2 to compensate.

Why would you need to compensate, as your pH during photoperiod seems perfectly stable at pH 5.1 ?

 

[Unexpected]

I would also vote for this, both for tank water and WC water. What you observe may or may not be CO2 related, and you may want to rule out some other substance that affects pH, or perhaps KH.

Ill check it again. But why would it change when running 0TDS water? I add no KH.

Why would you need to compensate, as your pH during photoperiod seems perfectly stable at pH 5.1 ?

I don't know, but if resting pH was lower, I assumed I needed to push lower to compensate. And indeed, 4.8 caused no stress where 4.97 sent my fish to the surface before this strange event.

both for tank water and WC water

I can't pH my water change water as TDS is 85 or so.

 

[Yugang]

Ill check it again. But why would it change when running 0TDS water? I add no KH.

If something in your tank would gradually increase KH, then that would lead potentially to an increase in pH (as you observe). I don't say this is what's happening, but a KH test is a quick experiment to rule potential explanations out.

I also like the idea to degas tank water just after WC, and a few days later to see if there is a pH difference. If so, then the explanation is not likely to be CO2 related.

I don't know, but if resting pH was lower, I assumed I needed to push lower to compensate.

When using pH as a proxy for CO2 ppm, the relevant pH drop is as compared to fully outgassed water pH. Tanks don't usually fully outgas overnight, but that is not relevant for the settings of your pH controller.

 

[Unexpected]

I have no clue what’s happening. Something must have gone on with the incoming water or because I chopped the plants down. Either way, pH started dropping long before the CO2 turned on so I went ahead and dosed .25dKH early this morning. Tank went to a 6.4 and set for a 1pH drop from there.

 

[Yugang]

dosed .25dKH early this morning

Have you actually measured KH, or a profile of KH over time? Dosing KH without measuring imo is not a good idea when using a pH/CO2 controller. Remember, the controller uses pH as a proxy for CO2, but this will fail if KH is not stable and you are likely to introduce CO2 variations into your tanks even if your pH profiles look perfect.

 

[Unexpected]

Have you actually measured KH, or a profile of KH over time? Dosing KH without measuring imo is not a good idea when using a pH/CO2 controller. Remember, the controller uses pH as a proxy for CO2, but this will fail if KH is not stable and you are likely to introduce CO2 variations into your tanks even if your pH profiles look perfect.

I’ve been running 0 but have seen a small pH rise throughout the week and assumed there’s a small amount coming from the substrate. My test kit doesn’t have enough resolution to even test for such a low amount.

Hopefully next water change resolves whatever is going on. If not, I’ll just dose .25 weekly. My low pH yesterday was 4.8 and the fish we’re getting twitchy.

 

[Yugang]

My test kit doesn’t have enough resolution to even test for such a low amount.

The test kit resolution can be increased by using a multiple (3-5) of the prescribed (5 cc?) tank water in the test tube, add the reagent drops and then divide the number of reagents drops by the same multiple to get the KH. I usually measure with 20 CC (4 * 5cc) tank water in a small bowl with white bottom, and divide the number of drops by 4 to get a 0.25 KH resolution. You don't need more precision than that.

My low pH yesterday was 4.8 and the fish we’re getting twitchy.

Probably because you're not using the fully outgassed pH as the reference for your pH controller.

I’ll just dose .25 weekly

I am not convinced this makes sense, or is perhaps jumping to a conclusion. I would measure KH, and how it changes over time, and from there decide if KH is indeed the root cause, if the variations explain your pH dropping, and if this was a temporal problem or needs to be addressed structurally.


Note: When deciding how much KH variation would be acceptable, I would use the KH/pH/CO2 table. When KH near 0, obviously small KH variations lead to rather big fluctuations in calculated CO2 ppm, this may be what you're observing in your tank.

Note: My WC tapwater KH is usually around 2, and for several years I have adjusted that with baking soda to 3 or 4 as that's what I understood from very old @plantbrain postings (which I may have misinterpreted, or to which he may not longer subscribe) was the best. I am not longer doing that, but realise from @Unexpected discussion that when tanks with RO water (0 dKH) have soil/hardscape even a little bit affecting KH, and using a pH/CO2 controller, it may be safer to adjust KH to slightly higher levels to minimise controller fluctuations due to these minor KH variations.

 

[Unexpected]

I took a sample of the water prior to adding KH and it was zero KH (just tested using your method) and degassed pH of that sample was 5.7. When I dropped the tank to 4.8, I saw no CO2 stress, just the twitching behavior as if their skin was irritated. So if I went off 5.7, that's going to be too low for these fish to be comfortable.
Per your method, I tested the tank water now and the KH is either .25 or .5, I don't know my exact gallons so between my eyes and probably wrong on true volume, I consider this matches my dose I gave the tank this morning. I'll see what the degassed pH is later, sample shaken and sitting now.

When I started your reactor, I never tested KH as my RO/DI water is always 0-1ppm, degassed was 6.5 back then and ran the tank off that number.

 

[Freshflora]

Another option is to stop using your PH controller to control CO2. At least temporarily. If you dial in a good injection rate, then regardless of KH/PH shifts (which are probably more common than not in tanks) you won’t have to worry about CO2 nearly as much. I feel like for most people using a controller causes too much of a headache because you have to be certain your KH is completely stable. Case in point, you’ve adjusted your CO2 a bunch in the last week already, which could easily trigger some algae. Most of the best plant growers don’t use PH controllers. At the end of the day, you have to adjust CO2 based on livestock and plant health/algae, and the relative PH drop method is just to get you in a good ballpark/starting point.

 

[Unexpected]

Another option is to stop using your PH controller to control CO2. At least temporarily. If you dial in a good injection rate, then regardless of KH/PH shifts (which are probably more common than not in tanks) you won’t have to worry about CO2 nearly as much. I feel like for most people using a controller causes too much of a headache because you have to be certain your KH is completely stable. Case in point, you’ve adjusted your CO2 a bunch in the last week already, which could easily trigger some algae. Most of the best plant growers don’t use PH controllers. At the end of the day, you have to adjust CO2 based on livestock and plant health/algae, and the relative PH drop method is just to get you in a good ballpark/starting point.

That’s a good idea. I think I’m going to do a 90% water change Wednesday and see if it sets things back to normal. Maybe pulling all the plants up and vacuuming threw a ton of organics into the water. Maybe this is the source of my troubles.

 

[FrankZ]

This is a very interesting observation. I might have an explanation but one that doesn’t really provide a solution beyond what @Freshflora mentioned regarding ditching the pH for controlling CO2.

In my line of work I have to accurately measure pH of various water sources. High purity RO/DI, steam condensate, waste streams, cooling towers, boilers etc. all with varying levels of ionic strength. For the ultra pure water sources, I need to have a dedicated pH probe calibrated with low ionic strength buffers.

Ionic strength effects the pH, google Henderson–Hasselbalch equation if you want a brain cramp for a while. It’s been over 20 years since I had to actually use that stuff (thanks advanced analytical chemistry) so it would take me a while to explain it properly.

Most readily available pH buffers are high ionic strength. Measure the TDS of your buffers and compare it to your tank water. You should see a large difference. That large difference can lead to you pH probe calibration being way off from the true pH.

That in and of itself isn’t a problem. As long as the curve is wrong at the top and bottom the pH controller should be close enough. The problem arises as things change over the course of a week. Rising or falling TDS charges the ionic strength of the tank and therefore changing the pH the controller is calculating. For non RO/DI tanks it probably isn’t noticeable but for tanks with minimal reconstitution and lean dosing it could be a large enough change to throw off the pH and thus your set point. I did see you use RO/DI but didn’t see your dosing/reconstitution amounts.

Seneye link this explains it well without the chemistry mumbo-jumbo I threw in above. It has some marketing stuff in it but it’s a decent explanation.

I could be way off on all of this explanation but I wanted to mention it. I did notice with my tank since I stopped using my pH controller and just injecting at a constant rate and only monitor the pH with the controller plant health and fish behavior has improved significantly.

 

[Unexpected]

This is a very interesting observation. I might have an explanation but one that doesn’t really provide a solution beyond what @Freshflora mentioned regarding ditching the pH for controlling CO2.

In my line of work I have to accurately measure pH of various water sources. High purity RO/DI, steam condensate, waste streams, cooling towers, boilers etc. all with varying levels of ionic strength. For the ultra pure water sources, I need to have a dedicated pH probe calibrated with low ionic strength buffers.

Ionic strength effects the pH, google Henderson–Hasselbalch equation if you want a brain cramp for a while. It’s been over 20 years since I had to actually use that stuff (thanks advanced analytical chemistry) so it would take me a while to explain it properly.

Most readily available pH buffers are high ionic strength. Measure the TDS of your buffers and compare it to your tank water. You should see a large difference. That large difference can lead to you pH probe calibration being way off from the true pH.

That in and of itself isn’t a problem. As long as the curve is wrong at the top and bottom the pH controller should be close enough. The problem arises as things change over the course of a week. Rising or falling TDS charges the ionic strength of the tank and therefore changing the pH the controller is calculating. For non RO/DI tanks it probably isn’t noticeable but for tanks with minimal reconstitution and lean dosing it could be a large enough change to throw off the pH and thus your set point. I did see you use RO/DI but didn’t see your dosing/reconstitution amounts.

Seneye link this explains it well without the chemistry mumbo-jumbo I threw in above. It has some marketing stuff in it but it’s a decent explanation.

I could be way off on all of this explanation but I wanted to mention it. I did notice with my tank since I stopped using my pH controller and just injecting at a constant rate and only monitor the pH with the controller plant health and fish behavior has improved significantly.

Thank you, I may go this route. It also seems the easier route in the end. Just let the reactor do it’s thing and just monitor pH from the controller.

I go 24ppm Ca and 8ppm magnesium, I believe that’s 3:1. I front load my macros also. 30,10,35