Having difficulty hitting the right CO2 concentration

Hi guys! It's my first time doing a high-tech tank and I need some advice regarding the adequate CO2 concentration for my setup (1 week old). Livestock has not been added to the tank yet.




Hardware:
Tank: 90x30x30cm (81L)
Effective water volume: ~60L
Hardscape: Spider wood, seiryu rocks, tropica aquasoil in filter bags capped with ADA sand
Lighting: Week Aqua P900 at 30% intensity for 8 hours a day
CO2: Too many bubbles to count, but it gets turned on 1 hour before the lights go up and stops one hour before lights retire
Filtration: Oase biomaster 350

Plants:

Spoiler

Background:
Pogostemon erectus
Rotala macrandra
Alternanthera reineckii ‘Rosanervig’
Rotala green

Midground:
Anubias nana
Anubias barteri
Anubias nana ‘golden’
Eriocaulon sp Vietnam
Bucephalandra ‘green broad leaf’
Bucephalandra ‘velvet’
Bucephalandra ‘kadagang’
Cryptocoryne lutea ‘hobbit’
Cryptocoryne balansae
Echinodorus ‘reni’
Staurogyne repens
Hygrophila pinnatifida
Hygrophila polysperma ‘rosanervig’
Hydrocotyle tripartita

Foreground:
Marsilea crenata

Water parameters:
I'm using a cheap pH meter and API test kit to test for pH.

Aerated tank water (bubbled overnight)
pH (meter): 8.33
pH (API): LR7.6, HR8.4
GH: 8
KH: 8

Fresh tank water before CO2 injection
pH (meter): 7.67
pH (API): LR7.2, HR7.4
GH: 8
KH: 3

CO2 injection pH stats
CO2 gets turned on 1 hour before lights go up. So at the 1 hour reading, that's when the lights just start.

1 hour: 6.6 (API), 6.84 (Meter)
2 hours: 6.4 (API), 6.64 (Meter)
3 hours: 6.0 (API), 6.63 (Meter) <- weird that API registered such a change while the meter did not
Will be updated~

Optimal CO2 concentration based on pH drop
What I understand is that the optimal CO2 concentration is achieved with a pH drop of 1. But which starting pH should I be looking at? Fresh tank water or aerated tank water?

What should be the pH my tank water should stabilize at during CO2 injection hours, based on the above information about my water parameters?

 

I suspect that you don't need to go below a pH of 7 and in an 80L tank I wouldn't expect that you'd need a constant stream of CO2 bubbles, you should be able to count them. Are you getting good diffusion and circulation? I see you have a drop checker, what colour is it after several hours of CO2 injection?

 

I'd go with the kH value of "bubbled overnight". I found a CO2 chart for you: CO2/pH/KH table
Just take your kH to find the desired pH window for optimal CO2. For example, for a kH of 8, shoot for a pH of 7.0 to 7.3.

 

I suspect that you don't need to go below a pH of 7 and in an 80L tank I wouldn't expect that you'd need a constant stream of CO2 bubbles, you should be able to count them. Are you getting good diffusion and circulation? I see you have a drop checker, what colour is it after several hours of CO2 injection?

Yes, the co2 bubbles are spread throughout the tank.

I managed to lower the co2 injection rate to around 3 bubble per second and here are my pH readings (de-gassed water pH is 8.33 (meter) and 8.2 (API):
1 hour - 7.24 (meter), 6.6 (API)
2 hours - 7.1 (meter), 6.6 (API)
3 hours - 6.97 (meter), 6.4 (API)
4 hours - 6.95 (Meter), 6.4 (API)

Its interesting that both meter and API readings agree with each other at higher pH (>8), but at lower pH they don't.

I'd go with the kH value of "bubbled overnight". I found a CO2 chart for you: CO2/pH/KH table
Just take your kH to find the desired pH window for optimal CO2. For example, for a kH of 8, shoot for a pH of 7.0 to 7.3.

I managed to lower the co2 injection rate to around 3 bubble per second and here are my pH readings (de-gassed water pH is 8.33 (meter) and 8.2 (API):
1 hour - 7.24 (meter), 6.6 (API)
2 hours - 7.1 (meter), 6.6 (API)
3 hours - 6.97 (meter), 6.4 (API)
4 hours - 6.95 (Meter), 6.4 (API)
7 hours - 6.92 (Meter)

Its interesting that both meter and API readings agree with each other at higher pH (>8), but at lower pH they don't.

 

The API low pH test is terrible for anything but relative measurements. It can tell you if pH is moving but not what it is with precision.

Looks like you are hitting a good high concentration according to the charts >= 25ppm. Any more than that could be hard on fish.