I'm new to pools, but not water chemistry. I have been a marine aquarium hobbyist for a few years. We care about the Alkalinity/Calcium/Magnesium balance, but just basically push the SI much higher to satisfy the needs of corals and snails to easily build themselves (making Calcium and Magnesium easy to deposit). There are some pretty solid articles from chemists in the aquarium community, and the consensus of most aquarists would say "Don't Chase pH". What that means is... Your pH is the end result of a complex water chemistry equation. pH can't tell you what to "fix", it's not an ingredient it's the end product. That said pH change is great to tell you to look for a problem in your system. Really for us aquarists, pH mostly tells how much CO2 is in the water. Because we already know (from testing) our Alk/Cal/Mag and we better not have any appreciable Borate/metals. So assuming atmospheric CO2 levels, we can calculate exactly what our pH should be. If that doesn't match our measured pH it means one of three things (in order of probability).
#1. You have more or less CO2 in your water relative to free air atmosphere equilibrium (95%),
#2. Your pH test is wrong, (4.9%),
#3 There is something really bad getting in your tank that you don't know is happening (0.1%).
If it isn't one of those things, congratulations you have officially broken chemistry. So if you have a properly calibrated pH probe (forget a color chart test that's a rough guess), it basically must be your CO2 level. Now here is the cool part, I can watch it happen in real time. My aquarium controller logs pH every minute. I can look at the chart and there is a daily cycle. As photosynthesis ramps up in the morning the available CO2 in the water drops, this causes pH to rise. At night the photosynthesis stops, but the fish are still making CO2 (breathing) and the pH drops. It looks like a sine wave every day. But it gets better! I can actually tell when people get home, or especially if the gas stove is in use. This drives atmospheric CO2 up in our house and the gas exchange drives it into the water also, again lowering pH. Keep in mind gas exchange is an equilibrium! With maximum water to air surface contact, the air and water will balance. You are never pumping an unlimited supply of carbonic acid (CO2 in water) into the water. It's not like putting in additional acid. It just equalizes with the air given enough gas exchange. If I had a party of 10 people and used the oven I would see a big pH drop in my aquarium. After the party and few hours for fresh air gas exchange my system will return to the exact same state as before, without any intervention from me. Because the air inside had returned to normal CO2 levels and the water equalized it's CO2 at that level.
I think you can see why for aquarists "adjusting pH" is dangerous. It will only be temporary and it will have negative consequences to ideal Alk/Cal/Mag levels. If you have a legitimate Alk/Cal/Mag problem, then fix that. If you have a CO2 problem, fix that. Not one of those two? It's another substance or a test error. Don't chase pH because then you are basically plugging random numbers into the Alk/Cal/Mag equation in order to look like you have the pH answer you want. Stability is our friend and nothing good happens fast in an aquarium.
Hopefully someone found that interesting, but I'd really like to explore how it might apply differently to pools.
Scenario 1: I have an algae outbreak, help! My TA/CH is OK (100/280), but my pH is high. Should I try to bring pH down before I add shock?
Answer A: Yes, the chlorine is more effective at lower pH and you should get pH adjusted first. This will obviously mess up your alkalinity, so you will need to fix that after the shock. But to actually kill the algae you need a lower pH.
Answer B: No, if you have algae it is consuming CO2, thus raising your pH somewhat artificially. Kill the algae and promote gas exchange (air injection or surface turbulence). Don't chase the pH, it will resolve itself if your TA/CH don't change.
Answer C: ?
Scenario 2: Help I can't get my pH down, my TA is 130 and my CH is 150, but my pH is 8.2. I'm adding acid and can't keep it down.
Answer A: Some pools just do that, keep adding acid.
Answer B: One of your tests is wrong, probably pH. Unless you have an algae bloom or something like a ton of Borate, that's impossible. You would need around TA 170+ and CH of 400+ to get close to that pH of 8.2. Do you have a way to verify your pH test (a reference solution)? Is there anything odd that happened recently and could be effecting tests? Some pools leach/gain carbonates into the water, and that will constantly push TA up, thus raising pH but if that were the case you would see TA rise between acid corrections.
Answer C: ?
Hopefully I didn't walk in a new room and start a battle royal. I still a have some questions about CYA and Chlorine chemistry, and I'm a totally new to trying to destroy all life in water. Aquarium or pool, I just don't see pH as anything other than a quick check that tells me I might have a problem, but I also don't trust any pH tests enough to use it for fixing any problem. I'd love to hear any ideas why this approach is wrong, and I'll be glad to learn something and admit it is wrong for pools.
#1. You have more or less CO2 in your water relative to free air atmosphere equilibrium (95%),
#2. Your pH test is wrong, (4.9%),
#3 There is something really bad getting in your tank that you don't know is happening (0.1%).
If it isn't one of those things, congratulations you have officially broken chemistry. So if you have a properly calibrated pH probe (forget a color chart test that's a rough guess), it basically must be your CO2 level. Now here is the cool part, I can watch it happen in real time. My aquarium controller logs pH every minute. I can look at the chart and there is a daily cycle. As photosynthesis ramps up in the morning the available CO2 in the water drops, this causes pH to rise. At night the photosynthesis stops, but the fish are still making CO2 (breathing) and the pH drops. It looks like a sine wave every day. But it gets better! I can actually tell when people get home, or especially if the gas stove is in use. This drives atmospheric CO2 up in our house and the gas exchange drives it into the water also, again lowering pH. Keep in mind gas exchange is an equilibrium! With maximum water to air surface contact, the air and water will balance. You are never pumping an unlimited supply of carbonic acid (CO2 in water) into the water. It's not like putting in additional acid. It just equalizes with the air given enough gas exchange. If I had a party of 10 people and used the oven I would see a big pH drop in my aquarium. After the party and few hours for fresh air gas exchange my system will return to the exact same state as before, without any intervention from me. Because the air inside had returned to normal CO2 levels and the water equalized it's CO2 at that level.
I think you can see why for aquarists "adjusting pH" is dangerous. It will only be temporary and it will have negative consequences to ideal Alk/Cal/Mag levels. If you have a legitimate Alk/Cal/Mag problem, then fix that. If you have a CO2 problem, fix that. Not one of those two? It's another substance or a test error. Don't chase pH because then you are basically plugging random numbers into the Alk/Cal/Mag equation in order to look like you have the pH answer you want. Stability is our friend and nothing good happens fast in an aquarium.
Hopefully someone found that interesting, but I'd really like to explore how it might apply differently to pools.
Scenario 1: I have an algae outbreak, help! My TA/CH is OK (100/280), but my pH is high. Should I try to bring pH down before I add shock?
Answer A: Yes, the chlorine is more effective at lower pH and you should get pH adjusted first. This will obviously mess up your alkalinity, so you will need to fix that after the shock. But to actually kill the algae you need a lower pH.
Answer B: No, if you have algae it is consuming CO2, thus raising your pH somewhat artificially. Kill the algae and promote gas exchange (air injection or surface turbulence). Don't chase the pH, it will resolve itself if your TA/CH don't change.
Answer C: ?
Scenario 2: Help I can't get my pH down, my TA is 130 and my CH is 150, but my pH is 8.2. I'm adding acid and can't keep it down.
Answer A: Some pools just do that, keep adding acid.
Answer B: One of your tests is wrong, probably pH. Unless you have an algae bloom or something like a ton of Borate, that's impossible. You would need around TA 170+ and CH of 400+ to get close to that pH of 8.2. Do you have a way to verify your pH test (a reference solution)? Is there anything odd that happened recently and could be effecting tests? Some pools leach/gain carbonates into the water, and that will constantly push TA up, thus raising pH but if that were the case you would see TA rise between acid corrections.
Answer C: ?
Hopefully I didn't walk in a new room and start a battle royal. I still a have some questions about CYA and Chlorine chemistry, and I'm a totally new to trying to destroy all life in water. Aquarium or pool, I just don't see pH as anything other than a quick check that tells me I might have a problem, but I also don't trust any pH tests enough to use it for fixing any problem. I'd love to hear any ideas why this approach is wrong, and I'll be glad to learn something and admit it is wrong for pools.