Aquarium CO₂ & Planted Tank Calculator

Planted Aquarium CO₂ Guide

What's the key thing to understand about The pH-KH-CO₂ Relationship?

In aquariums, the carbonate buffering system links pH, KH (carbonate hardness), and dissolved CO₂. The relationship: CO₂ (mg/L) = 3.0 × KH × 10^(7 − pH). This formula assumes CO₂ is the only acid in the water — accuracy decreases if other acids (phosphoric, organic acids) are present. Ideal planted tank CO₂: 20-30 mg/L. Below 10 mg/L: plants cannot photosynthesise efficiently. Above 40 mg/L: risk of fish stress and death. A KH of 5 and pH of 7.0 gives CO₂ ≈ 15 mg/L — borderline adequate for plants.

What should I know about Injecting CO₂ for Plant Growth?

Planted aquariums with medium to high light require CO₂ supplementation to prevent deficiencies. Three methods: pressurised CO₂ (cylinder + regulator + diffuser) — most reliable and controllable, recommended for serious planted tanks. DIY fermentation (yeast + sugar in a bottle) — cheap and effective for small tanks, but hard to control CO₂ levels precisely. Liquid CO₂, such as Seachem Excel or Easy-Life EasyCarbo, is a convenient supplement, but provides only a small fraction of what pressurised systems deliver, and works best in low-tech or lightly planted tanks rather than as a genuine substitute for a full pressurised CO₂ system in a heavily planted setup.

What should I know about KH and pH Stability?

Carbonate hardness (KH) buffers the aquarium against pH swings. Low KH (under 3 dKH) means pH can fluctuate dramatically throughout the day as plants consume CO₂ during the photoperiod and CO₂ builds up at night. Minimum KH for a stable planted tank: 3-4 dKH. If your tap water has very low KH, add sodium bicarbonate (baking soda) to raise it — 1 teaspoon per 100 litres raises KH by approximately 4 dKH. For a low-pH CO₂ tank: avoid raising KH too high, as this makes achieving the target CO₂ pH harder, since a higher KH buffers the water more strongly against the pH drop CO₂ injection is meant to produce.

What should I know about CO₂ Safety for Fish?

Signs of CO₂ overdose in fish: gasping at the surface (seeking oxygen), rapid gill movement, erratic swimming, loss of colour, gathering near the filter outlet. CO₂ above 40 ppm is stressful for most fish; above 50-60 ppm can be fatal. Safe approach: start CO₂ 1-2 hours after lights on (plants consuming CO₂ and producing O₂ rather than consuming O₂). Stop CO₂ 1-2 hours before lights off. Never run CO₂ at night. Install a drop checker (colour indicator) or pH controller with an automated shut-off to stop injection automatically if levels climb too high overnight.

Why Does the Relationship Between pH, KH, and CO2 Matter for Safe Injection?

In water with carbonate hardness (KH), dissolved CO2 reacts with water to form carbonic acid, which lowers pH in a mathematically predictable relationship with KH — this is why aquarists injecting CO2 for planted tank growth use pH and KH readings together (via a standard reference chart or calculation) to estimate actual dissolved CO2 concentration, since CO2 itself is difficult to measure directly in a home aquarium. This relationship is also the basis of the widely used 'pH drop checker,' a small device containing reference fluid that changes colour based on CO2 concentration, giving a visual real-time indicator (commonly green indicating a safe, adequate CO2 level around 30ppm, blue indicating too little, yellow indicating dangerously high) that aquarists use to monitor and adjust CO2 injection rate without needing to calculate the pH/KH relationship manually for every check.

Why Does CO2 Injection Require Careful Timing Relative to the Tank's Lighting Schedule?

Plants only consume CO2 and produce oxygen during active photosynthesis, which requires light, meaning CO2 injected while the tank lights are off is not being used by plants and simply accumulates in the water, potentially reaching levels that are safe for plants but increasingly stressful or dangerous for fish, which continuously need dissolved oxygen and are more vulnerable to elevated CO2 overnight when photosynthesis isn't counteracting it. This is why CO2 injection systems are typically set on a timer to start an hour or two before the main lighting period begins (allowing CO2 levels to build to an effective concentration right as photosynthesis ramps up) and to stop before or as the lights turn off, rather than running continuously around the clock, which both wastes CO2 during dark hours and creates unnecessary risk to fish overnight when plants are not consuming it.

What should I know about Signs of CO2 Overdose in Fish?

Because CO2 injection for planted tanks directly affects water chemistry that fish also depend on, overdosing is a real risk that owners should know how to recognize quickly. Early signs include fish gasping at the surface, clamped or rapid gill movement, and unusual lethargy, all suggesting the fish are struggling to extract enough oxygen from water that's become too acidic or oxygen-poor relative to its CO2 content. Because these signs can escalate quickly, many keepers run an air stone or increase surface agitation specifically overnight, when CO2 injection combined with plants' lack of photosynthesis (and therefore oxygen production) in darkness creates the highest-risk period for oxygen depletion. Timers that turn off CO2 injection an hour or so before lights-out, rather than running it continuously around the clock, are the standard way to avoid this overnight risk window.

Worked Example: Estimating CO₂ Concentration from pH and KH

Planted-tank keepers commonly estimate dissolved CO2 from pH and carbonate hardness (KH) using the approximation CO2 (ppm) ≈ 3 x KH (dKH) x 10^(7 - pH). This comes from the chemical relationship between carbonic acid, bicarbonate and pH in water, and is the same logic this calculator applies. Worked example: a tank with KH = 4 dKH and pH = 6.8 gives CO2 ≈ 3 x 4 x 10^(7 - 6.8) = 12 x 10^0.2 ≈ 12 x 1.585 ≈ 19 ppm, which sits in the healthy 15-30ppm range most aquatic plants use efficiently without stressing fish. If the same tank's pH dropped to 6.4 with KH unchanged, CO2 would rise to roughly 12 x 10^0.6 ≈ 12 x 3.98 ≈ 48 ppm, which is high enough to risk fish distress, showing how quickly CO2 rises as pH falls at a fixed KH.

Using a Drop Checker to Monitor CO₂ Day to Day

Because pH and KH readings only give a snapshot, many planted-tank keepers use a drop checker: a small glass bulb containing a pH-indicator solution (often mixed with 4 dKH reference fluid) that sits inside the tank and slowly equilibrates with the water's dissolved CO2 through an air gap. The indicator's colour, usually read against a blue-green-yellow chart, shows roughly whether CO2 is too low (blue), in a safe working range (green), or getting high enough to risk fish (yellow), with a lag of 1-2 hours behind the tank's real-time level. A drop checker is not a substitute for watching fish behaviour directly; fish gasping at the surface, staying unusually still, or clamped fins are more immediate warning signs than any chemical test, and CO2 injection should be reduced or paused immediately if they appear.

Balancing CO₂ with Light Intensity and Plant Mass

CO2 injection works together with lighting rather than independently of it, since plants only use CO2 at a meaningful rate while actively photosynthesising under adequate light, and injecting CO2 into a dimly lit or sparsely planted tank mostly just raises the risk to fish without much corresponding benefit to plant growth. A common approach is to start CO2 injection roughly 1-2 hours before lights come on, so levels are already in the target range when plants begin actively using it, and to turn injection off an hour or so before lights go off, since plants stop consuming CO2 once photosynthesis stops for the day and unused CO2 would otherwise continue accumulating overnight when fish most need stable, undiminished oxygen levels.

What to Do if CO₂ Rises Too Quickly

If a drop checker turns yellow, fish start gasping at the surface, or a pH test shows CO2 has climbed well above the target range, the first response should be to immediately stop CO2 injection and increase surface agitation or add an air stone, since agitating the surface drives dissolved CO2 out of the water faster than it would escape on its own. Reducing the bubble rate for future sessions, rather than simply restarting at the same rate once fish recover, is the longer-term fix, since a setup that reached dangerous levels once is likely to do so again under similar conditions such as a particularly still, warm day. Checking fish behaviour at the point in the day CO2 is highest, typically late in the lighting period, is a useful habit for catching a rate that's set slightly too high before it becomes a repeated problem.