I wrote this post some months ago (Feb 2017, I think), but never published it. The measurements will probably be different now, as I’ve made many changes to the tank since then. However, the general conclusion (for my tank) should be the same: CO₂ levels remain pretty stable once they reach peak levels.

Since then, I have recently found extra information that explains why my CO₂ is stable and not fluctuating (as far as I can tell). I’ll mention that at the end of the post. Everything in the quote block below was my original post.

I had read in various places on the internet that the gases that accumulated in the Cerges CO₂ Reactor in the middle of the day could be a combination of injected CO₂ and saturated O₂ released from the water. This gas accumulation could hamper the efficiency of the reactor.

From previous pH measurements, I know that my tank has enough CO₂ at the time of measurement. I decided to check if the levels drop when the gases accumulate.

I decided to measure the pH levels in the tank hourly from the time my CO₂ solenoid turns on till a few hours after the gases start to accumulate in the reactor. I started a little later than intended, but the impact is minimal as the purpose was to observe the consistency of CO₂ levels throughout the day. The table below shows the measurements:

Hours Passed	Time	pH	CO2 (ppm)	Gas Accumulation (By estimation of water level in reactor)
0	1038	7.3	9.0	Not observable
1	1100	7.1	14.3	Not observable
2	1200	6.9	22.7	Not observable
3	1300	6.7	36.9	Not observable
4	1400	6.7	36.9	Approximately 1 inch above transparent reactor body (Estimated by movement of bio-balls.)
5	1500	6.6	45.2	Approximately 0.5 inch above transparent reactor body
6	1600	6.6	45.2	Approximately 0.25 inch above transparent reactor body
7	1700	6.6	45.2	Slightly above transparent reactor body

Notes:

• KH is 6 degrees.
• CO₂ ppm levels are derived from this CO₂-KH-pH Chart at BarrReport.com.
• BPS is 7.4. The bubble counter is filled with glycerine instead of water.

Observations:

1. CO₂ level reached an acceptable 20 ppm at about 2 hours, which is 1 hour into my lighting period.
2. CO₂ level reached the desired 30 ppm at 3 hours.
3. CO₂ level peaked at 45.2 ppm at 5 hours.
4. Gas probably started to accumulate in the reactor from about the 3rd hour.
5. CO₂ level continued to rise despite the start of gas accumulation.
6. CO₂ level consistent at 45.2 ppm during the last 3 hours despite gas accumulation.
7. pH dropped by 0.2 units per hour for the first 3 hours.
8. pH only dropped by 0.1 units per hour after the first 3 hours.

Analysis:

At that BPS, it takes about 3 hours for CO₂ level to reach the desired 30ppm. Gas also starts to accumulate from the 3rd hour. But CO₂ level continues to rise and only peaks after 5 hours, which is 4 hours into the lighting hours.

Despite the significant accumulation of gases, it does not appear to cause CO₂ levels to drop or fluctuate. pH started to drop slower after gas accumulation started, but this is expected. If the water is reaching CO₂ saturation, CO₂ will not dissolve easily and start to accumulate. pH is not a linear scale, so at lower pH, the same amount of additional CO₂ actually causes a smaller drop in pH.

The CO₂ level takes too long to peak, but reach acceptable 20+ ppm in 2 hours and the desired 30+ ppm in 3 hours.

After-Action:

Based on the peak of 45 ppm of CO₂ above, I decided to lower my BPS to about 6, to see if I can peak at a lower CO₂ ppm. Most likely, it’ll probably take longer to build up CO₂ which turned out to be true. The next day, I measured the pH at 2pm and the reading was only 6.9 compared to 6.7 the day before. It’s not yet the desired level of CO₂ , and it already took too long to build up to that level.

Back to the present time.

According to the chart, I am actually injecting a lot of CO₂. Much higher than the desired 30ppm to 35ppm I was aiming for. At this rate, I am going through one 5-litre cylinder of CO₂ every month. It would be nice to lower that.

However, I recently discovered an alternative method to measure CO₂ levels in the tank that ignores the pH-KH-CO₂ chart. That chart is based on the assumption that carbonates are the only thing affecting your tank KH. The truth is most hobbyists have no idea and no easy/cheap way to find out what’s affecting our KH. The alternative method is to find out the pH of your tank water at ambient CO₂ levels and then aim for a 1pH drop to get 30 ppm CO2. Bigger pH drops for more CO2. There are many posts on the internet about this, but here’s one: Another CO₂ Chart to try.

So, here are my recent pH measurements:

• De-gassed tank water: 8.5pH
• Tank water a few hours of CO₂ injection: 6.8pH

According to the alternative method, ambient CO₂ levels in water are estimated at 2 to 3 ppm. If my de-gassed tank water is about 2ppm, with a 1.7pH drop, I am actually achieving around 90ppm of CO₂! If my water’s ambient CO₂ is at 3 ppm, then I’m achieving around 150 ppm!  I think both are way off! Another source suggests that ambient CO₂ levels are around 0.6 ppm. In that case, I have only about 30 ppm of CO₂. Not ideal, since it takes quite a while to build up to that. On the other hand, given the accuracy of hobby-grade pH meters and KH test kits, a difference of +/- 0.1 pH and +/- 0.5dKH could make a fair bit of difference in CO₂ ppm.

As it is, I don’t think I can push my CO₂ levels much higher since my reactor starts to accumulate CO₂ halfway through the injection period. For now, I’m going to keep my CO₂ injection rate as it is, since I could grow plants nicely before with the same reactor and similar injection rates.

Next, why am I getting stable CO₂ levels, if my reactor accumulates gas? First, watch this video by Dennis Huang: Optimizing CO₂ in a planted tank.

So, I am able to achieve approach A (about the 3:41 mark of the video, but watch from the start for the explanation): high injection rate, high off-gassing rate. A high injection rate is easy; just inject lots of gas. A high off-gassing rate is achieved because I use a sump for filtration. Furthermore, the gas building up in the reactor should be CO₂ according to this thread: What is the gas that builds up inside the Reactor during the day? This. The thread did not confirm it, but the deduction seems sound. The pump that runs my reactor stops 1 hour after I stop the CO₂ injection, and the built-up gas dissolves completely within that time. So the build-up may make the reactor less efficient, but it is still dissolving CO₂ only.

So, that concludes that my CO₂ levels are stable. CO₂ levels are sufficient since I was able to grow plants well and fast with more lights before. I might tweak to waste less CO₂ once I get my tank plants to grow well again, but at the moment I can rule out CO₂ issues causing my plant growth and algae issues.