I need help achieving a 1.0 pH drop

[Dechi]

Hi everyone,

I'm trying to fine-tune my CO2 injection setup to achieve the classic 1.0 pH drop for my planted tank, but I'm running into an issue where lowering my pH setpoint doesn't change the relative drop—it just shifts my overall pH baseline lower.

Tank & Equipment Setup​

• Tank Size: 135 gallons (48" x 24" x 22" high)
• Filtration & Circulation: Sump setup with a Vectra M2 return pump and an overflow box with two outlets
• CO2 Hardware: CO2 Art regulator with a 10 lb cylinder, feeding into an Ista Max Mix CO2 Reactor filled with bioballs (tube output directly under the return pump)
• Automation: Milwaukee MC122 PRO pH Controller on a timer (on at 0959 and off at 1700). It takes 30-45 minutes to reach the 0.7 ph drop
• Pressure is set to 30 psi and bubbles too fast to count but not continuous line

The Issue / Dilemma​

No matter where I set my target pH on the Milwaukee controller, I can't seem to get a full 1.0 pH reduction from my degassed/morning baseline.

• Example:
  • My morning pH (before CO2 turns on) is around 6.7 at the moment
  • The Milwaukee controller turns off at 6.0 (0.7 drop)
  • If I lower the Milwaukee setting to 5.9 to get a 0.8 drop instead of 0.7, the next morning the ph will now be 6.6 so the drop remains 0.7
  • I’ve tested it 3 times and it stays the same (0.7 drop) no matter what setting I try

It feels like a moving target where the morning starting pH always tracks down to match the controller's target pH, so the relative drop between "degassed" and "fully injected" never reaches 1.0 unit.

My Questions​

1. Why is this constant drift in morning pH happening ?
2. How do you properly calibrate a 1.0 pH drop when using a continuous pH controller ?
3. What could be causing this constant shift in ph overnight ?
   

Any insights, feedback, or troubleshooting steps from experienced planted tank hobbyists would be greatly appreciated!

Thanks!

 

[Rusty_Shackleford]

Have you tried turning on the CO2 earlier?
Perhaps you can get a lower drop before the plants start consuming CO² when the lights turn on.

 

[Dechi]

Have you tried turning on the CO2 earlier?
Perhaps you can get a lower drop before the plants start consuming CO² when the lights turn on.

I’m a beginner with co2, not sure I understand the logic behind it. What would it change ? PH has dropped 0.7 when the lights go on.

 

[Rusty_Shackleford]

Are you turning the CO2 off at night to de-gas?
It's pretty standard to turn on the CO2 anywhere between 2-3 hours before the lights go on. You're aiming to hit 30 ppm of CO2 by lights on. That should come close to your desired 1.0 drop in pH. When the lights come on the plants will start using CO2.
You're not noticing a rise in pH during the photoperiod correct? So it seems your system is able to keep a steady amount of CO2 as the plants use it during the day. That's good.
So by starting CO2 earlier than you currently are you should be able to hit your targeted pH drop.

 

[Rusty_Shackleford]

Another valuable piece of information is your actual degassed pH.
Take a cup of tank water out and let it sit for 24-36 hours, then measure the pH.
The reason I mention this is because the actual pH of your water may not be what you think it is and it's likely your tank is not fully degassed by the time your CO2 starts again in the morning.

 

[Dechi]

Are you turning the CO2 off at night to de-gas?

Yes, it comes off at 1700 and the lights go off at 1730. I have a powerhead aimed at the surface that comes on at 1800 for better oxygenation at night.

You're not noticing a rise in pH during the photoperiod correct?

The Milwaukee does it’s job and turns the co2 back on when ph is 0.1 to 0.2 degree above set point.

So by starting CO2 earlier than you currently are you should be able to hit your targeted pH drop.

Ok, I will try that and report back.

Another valuable piece of information is your actual degassed pH.

Degassed ph (after 2-3 days) from tap water is 7.4 and ph out of the tap is 6.8

 

[Rusty_Shackleford]

Degassed ph (after 2-3 days) from tap water is 7.4 and ph out of the tap is 6.8

What is degassed pH from the aquarium water?

 

[Dechi]

What is degassed pH from the aquarium water?

I will be checking that now. I’ll let you know in 2 days.

I’ve thought about injecting earlier but I don’t think it will change anything.

Here is why: I'm using a continuous pH controller (Milwaukee MC122) rather than a fixed timer and needle valve. The controller measures the pH in real-time and automatically shuts off the solenoid valve the exact moment it hits the target setpoint (for example, pH 6.0).

Because the controller enforces a hard stop at the target setpoint, starting the injection earlier simply means it reaches that target sooner—it won't push the pH or the CO2CO2 level any lower. The controller cuts off the gas regardless of how early it started.

 

[Rusty_Shackleford]

I will be checking that now. I’ll let you know in 2 days.

I’ve thought about injecting earlier but I don’t think it will change anything.

Here is why: I'm using a continuous pH controller (Milwaukee MC122) rather than a fixed timer and needle valve. The controller measures the pH in real-time and automatically shuts off the solenoid valve the exact moment it hits the target setpoint (for example, pH 6.0).

Because the controller enforces a hard stop at the target setpoint, starting the injection earlier simply means it reaches that target sooner—it won't push the pH or the CO2CO2 level any lower. The controller cuts off the gas regardless of how early it started.

I've never used a Milwaukee controller. Maybe it just needs to be recalibrated?

 

[Dechi]

I've never used a Milwaukee controller. Maybe it just needs to be recalibrated?

I’ve done it, even though it’s recent. But it’s working fine, keeping the ph where it’s set point is. The problem is degassing at night. But today I was able to get some pearling on some leaves so I think I’m getting there.

If I set the Milwaukee lower at 5.8, some fish go to the surface so it’s too much. I can’t go lower than 5.9.

 

[Rusty_Shackleford]

I’ve done it, even though it’s recent. But it’s working fine, keeping the ph where it’s set point is. The problem is degassing at night. But today I was able to get some pearling on some leaves so I think I’m getting there.

If I set the Milwaukee lower at 5.8, some fish go to the surface so it’s too much. I can’t go lower than 5.9.

Hmm.something is off. A majority of fish can handle 30 ppm of CO2. 45-50 ppm is where you start to see issues like going to the surface for oxygen.

 

[Dechi]

Hmm.something is off. A majority of fish can handle 30 ppm of CO2. 45-50 ppm is where you start to see issues like going to the surface for oxygen.

It might be me being paranoid. This is my first time using co2 and I want to make sure I don’t hurt my fish.

 

[coralbandit]

Are you using a drop checker also in the tank? Where is the Milwaukee's sensor located? In the tank or sump? What is your Kh?
I had no issues dropping 1 point in ph in my 180 with dual overflows and istas in my sump. I used a Milwaukee controller and it was great.

 

[Dechi]

Are you using a drop checker also in the tank?

Yes but it’s pretty useless. It gets to medium green, never light green.

Where is the Milwaukee's sensor located? In the tank or sump?

It’s in the sump, in the second compartment, before the return pump where the co2 tube is attached.

So the water gets co2 before hitting the return pump, goes all around the tank, gets down into the sump and the sensor reads the pH before it goes around again.

What is your Kh?

Last time I measured it (in 2021) it was about 100 out of the tap. I no longer have a test for it.

I had no issues dropping 1 point in ph in my 180 with dual overflows and istas in my sump. I used a Milwaukee controller and it was great.

What was your baseline coming and what was your set point ?

I think my rookie mistake is not using my degassed water from the tank as a baseline. My water doesn’t degass 100% during the night so in the morning, pH is 6,6 or 6,7 and Milwaukee setPoint is 5.9 or 6.0.

I was trying to get 1.0 down from 5.9 or 5.0 but it never works because the starting pH in the morning drops with the set point.

Tomorrow I’ll know my degassed value from the tank water. This should be my baseline, right ?

 

[Rusty_Shackleford]

Yes but it’s pretty useless. It gets to medium green, never light green.


It’s in the sump, in the second compartment, before the return pump where the co2 tube is attached.

So the water gets co2 before hitting the return pump, goes all around the tank, gets down into the sump and the sensor reads the pH before it goes around again.


Last time I measured it (in 2021) it was about 100 out of the tap. I no longer have a test for it.


What was your baseline coming and what was your set point ?

I think my rookie mistake is not using my degassed water from the tank as a baseline. My water doesn’t degass 100% during the night so in the morning, pH is 6,6 or 6,7 and Milwaukee setPoint is 5.9 or 6.0.

I was trying to get 1.0 down from 5.9 or 5.0 but it never works because the starting pH in the morning drops with the set point.

Tomorrow I’ll know my degassed value from the tank water. This should be my baseline, right ?

 

[Dechi]

So after 48 hours my degassed water from the two has a pH of 6.9 (6.7 directly from the tap).

So I adjusted the Milwaukee to 5.9 and this should give me a 1.0 drop from my degassed baseline. I am seeing a bit of pearling already so hopefully I’m on the right track !

Too bad it took me 3 months but hey, that’s what learning is all about.

 

[Rusty_Shackleford]

Yes, it comes off at 1700 and the lights go off at 1730. I have a powerhead aimed at the surface that comes on at 1800 for better oxygenation at night.

If it were me, I would probably shut off the CO2 earlier.
I don't have a citation for you, but I've seen other plant gurus shut off the CO2 a couple of hours before lights out. I've always done this as well. The reason is plants need less CO2 later in the day, during the "late afternoon" of their photoperiod. They don't photosynthesize as much late in the photoperiod as the do early in the photoperiod.

It’s in the sump, in the second compartment, before the return pump where the co2 tube is attached.

So the water gets co2 before hitting the return pump, goes all around the tank, gets down into the sump and the sensor reads the pH before it goes around again

This may have something to do with your readings and I missed it the first time.
By the time your water makes it back to the sump where your sensor is some CO2 has already been used by the plants plus off gassed due to surface agitation. I would be interested to see what the reading is if the sensor is in the aquarium.

 

[Dechi]

If it were me, I would probably shut off the CO2 earlier.
I don't have a citation for you, but I've seen other plant gurus shut off the CO2 a couple of hours before lights out. I've always done this as well. The reason is plants need less CO2 later in the day, during the "late afternoon" of their photoperiod. They don't photosynthesize as much late in the photoperiod as the do early in the photoperiod.

I’ll read up on that, thanks.

This may have something to do with your readings and I missed it the first time.
By the time your water makes it back to the sump where your sensor is some CO2 has already been used by the plants plus off gassed due to surface agitation. I would be interested to see what the reading is if the sensor is in the aquarium.

The pump is set to 1500 gallons per hour so it goes around fast but that’s a fair point and I’ll check tomorrow just to be safe.

 

[Rusty_Shackleford]

The pump is set to 1500 gallons per hour so it goes around fast but that’s a fair point and I’ll check tomorrow just to be safe.

What could it hurt?
We're problem solving right? It's worth checking out all the possibilities and eliminating potential issues one by one.
There are some sharp minds here, we'll get this figured out.

 

[Dechi]

I checked the pH from the tank and compared it with the pH from the sump and they are pretty much identical.

BUT… I cleaned the pH probe yesterday while checking my baseline and this morning the pH was 7,0 instead of 6,6 ! Which makes more sense.

So I think my system is in line now and working as it should. Now let’s wait for the results !

Here is the tank today so we can compare :