CO2, pH and KH: Understanding the Relationship
The relationship between CO2, pH and KH is one of the most important — and most misunderstood — concepts in planted aquarium keeping. Understanding how these three parameters interact allows you to accurately estimate your CO2 levels without expensive equipment, optimise plant growth and keep fish safe. This CO2 pH KH aquarium relationship guide from Gensou Aquascaping at 5 Everton Park breaks it down clearly.
The Basic Relationship
CO2 dissolved in water forms carbonic acid (H₂CO₃), which lowers pH. The more CO2 you inject, the lower your pH drops. KH (carbonate hardness) acts as a buffer — higher KH resists pH changes, meaning you need more CO2 to achieve the same pH drop. The three parameters are mathematically linked: if you know any two, you can calculate the third. This is the foundation of the CO2/KH/pH chart used throughout the hobby.
Reading the CO2/KH/pH Chart
The standard chart shows CO2 concentration (in ppm) at the intersection of your measured KH (in dKH) and pH values. For example, with KH 4 dKH and pH 6.6, the chart shows approximately 30 ppm CO2 — the target for most planted tanks. The green zone (20–35 ppm) is ideal for plant growth while remaining safe for fish. Yellow (below 20 ppm) means insufficient CO2 for demanding plants. Light blue (above 35 ppm) risks stressing fish.
Why This Matters Practically
A drop checker with bromothymol blue indicator solution gives you a rough visual indicator (green = good), but the CO2/KH/pH chart gives you an actual number. This is useful for fine-tuning — if your plants are growing well at 25 ppm, you know not to push further. If you are at 15 ppm and plants are struggling, you know exactly how much more CO2 is needed.
The Accuracy Problem
The chart assumes that the only acid affecting pH is carbonic acid from CO2. In reality, other acids are present — tannins from driftwood, organic acids from decomposing matter, and acids released by aqua soil. These additional acids lower pH independently of CO2, causing the chart to overestimate your actual CO2 level. A tank with lots of driftwood and aqua soil may show “30 ppm CO2” on the chart when actual CO2 is only 15 ppm. This is why experienced hobbyists use the chart as a guide, not gospel.
Practical Approach for Singapore Hobbyists
Singapore’s tap water is soft with low KH (typically 1–3 dKH). This means small amounts of CO2 cause large pH swings — your water has minimal buffering capacity. With KH 2 and CO2 injection, pH can easily drop from 7.0 to 6.0 or lower. This is fine for plants and most soft-water fish, but monitor carefully to avoid crashes below pH 5.5.
If using aqua soil (which further lowers KH), your effective KH may be close to zero, making pH extremely volatile. Some hobbyists add a small amount of crushed coral in the filter to maintain a minimum KH of 1–2 dKH as a safety net against dangerous pH crashes overnight when CO2 continues dissolving but plants stop consuming it.
Drop Checker vs Chart
Use both for the most accurate picture. The drop checker uses a reference KH solution (usually 4 dKH) isolated from your tank water, so tannins and other acids do not affect its reading. When the drop checker shows green and your pH/KH chart calculation also shows 20–30 ppm, you can be confident your CO2 is on target. If the two methods disagree, trust the drop checker — it is less affected by confounding acids.
Key Takeaways
Higher KH requires more CO2 to achieve the same concentration — if you raise KH, you may need to increase your bubble rate. Lower pH at the same KH means more CO2 — a useful diagnostic when plants are not growing well. The chart is a tool, not a truth — use it alongside a drop checker and observation of plant and fish behaviour. Your fish will tell you when CO2 is too high: gasping at the surface, rapid gill movement and lethargy are immediate signals to reduce injection or increase surface agitation.
Related Reading
emilynakatani
Still Have Questions About Your Tank?
Drop by Gensou Aquascaping — most walk-in questions get answered in under 10 minutes by someone who has set up hundreds of tanks.
5 Everton Park #01-34B, Singapore 080005 · Open daily 11am – 8pm
