Quick answer: Most planted tanks with injected CO2 aim for about 25 to 35 ppm, with 30 ppm as the usual target. The easiest reliable way to check is the pH drop method: CO2 running should lower your pH by about 1.0 compared with the same water left to degas. The classic KH and pH chart gives a number too, but it reads too high in tanks with aquarium soil, peat or driftwood. This calculator does all three checks, KH and pH, pH drop, and drop checker color, and tells you whether your level is safe for fish.
To get the degassed pH, leave a cup of tank water out for 24 hours, or bubble an air stone in it for an hour, then test it. Test the tank pH at the end of the CO2 period, when lights have been on a few hours.
Estimates only. CO2 levels change through the day, so measure at the same time each day and always watch your fish. Gasping at the surface means turn the CO2 down right away.
Three ways to measure CO2 in an aquarium
| Method | What you need | Accuracy | Best for |
|---|---|---|---|
| pH drop | A pH test kit or pen, a cup of tank water | Good in any tank | Soil tanks, wood heavy tanks, everyone really |
| KH and pH chart | KH test and pH test | Good only with inert substrate and no peat | Sand or gravel tanks with steady KH |
| Drop checker | A drop checker with 4 dKH fluid | Rough, about 2 hours behind | A quick daily glance |
Here’s the honest part most CO2 calculators skip. The KH and pH formula assumes carbonate is the only thing changing your pH. In a lot of planted tanks it isn’t. Aquarium soil, peat and driftwood release acids that drop pH on their own, so the chart thinks you have far more CO2 than you do. People then turn their CO2 down and wonder why plants stall. The pH drop method sidesteps all of that because it compares the same water with and without CO2.
How to use each method
KH and pH method. Test your KH and your pH near the end of the CO2 period, a few hours after lights on. Enter both, and pick your substrate. The calculator shows your estimated CO2 and the pH you’d need for about 30 ppm at your KH. The formula is CO2 = 3 x KH x 10 to the power of (7 minus pH).
pH drop method. Take a cup of tank water and leave it out for 24 hours, or run an air stone in it for an hour, so the extra CO2 escapes. Test that water. That’s your degassed pH. Then test the tank during the CO2 period. Every 1.0 drop in pH means roughly 10 times more CO2, so a 1.0 drop from a degassed level of about 3 ppm lands near 30 ppm.
Drop checker color. Fill the checker with 4 dKH reference fluid plus a pH indicator, and read the color. Blue means too little, green is the target, and yellow means too much. Remember the color shows the CO2 level from about two hours earlier.
CO2 levels and what they mean
| CO2 level | pH drop | Drop checker | What it means |
|---|---|---|---|
| Under 15 ppm | Under 0.7 | Blue | Low tech level. Fine without CO2 plants |
| 15 to 25 ppm | 0.7 to 0.9 | Blue green | Moderate. Good for medium light |
| 25 to 35 ppm | 0.9 to 1.05 | Green | The target for most high tech tanks |
| 35 to 45 ppm | 1.05 to 1.2 | Lime | High. Watch fish closely |
| Over 45 ppm | Over 1.2 | Yellow | Dangerous for many fish |
How to dial in CO2 safely
- Start low. Begin at 1 bubble per second for a 20 gallon and adjust from there. Bubble counts mean little between setups, so treat them as a starting point only.
- Change slowly. Raise the rate a little, then wait a full day before judging. Fish handle a slow rise far better than a sudden jump.
- Time it with your lights. Turn CO2 on 1 to 2 hours before lights on and off about an hour before lights off, ideally with a solenoid and timer.
- Measure at the peak. Test pH or read the checker near the end of the photoperiod, when CO2 is at its highest.
- Keep surface movement. A gentle ripple keeps oxygen up without blowing off all your CO2.
- Feed the plants too. More CO2 means plants use more nutrients, so pair it with a proper dosing plan from our aquarium fertilizer calculator.
Not sure of your real water volume? Our aquarium volume calculator works it out, which also helps you size your diffuser and reactor.
Common CO2 problems
Fish gasping at the surface. Too much CO2. Turn the regulator down, add an air stone right away, and do a water change if fish look badly stressed. Shrimp like Amano shrimp often show it first by climbing to the waterline.
Drop checker stays blue. Not enough CO2 reaching the water. Check for leaks with soapy water, move the diffuser under the filter outflow, and clean a clogged diffuser.
pH swings a lot every day. Normal with CO2. A 1.0 daily swing is expected and fish adapt to it. What hurts them is a sudden change, like a regulator dumping its bottle.
Plants still pale with CO2 on. Usually nutrients, not CO2. Check our plant deficiency symptoms guide.
Algae got worse after adding CO2. Often unstable CO2, either on too late or swinging day to day. Aim for steady levels at the same time every day. Black beard algae in particular loves fluctuating CO2.
No CO2 at all. That’s fine. Plenty of tanks grow beautifully without it. Our low light aquarium plants list and dwarf sagittaria guide cover plants that don’t need it.
FAQ
How much CO2 should be in a planted aquarium?
About 25 to 35 ppm for high tech tanks with strong light, with 30 ppm as the usual target. Low tech tanks without injection naturally sit around 2 to 5 ppm.
How do I calculate CO2 from KH and pH?
CO2 in ppm = 3 x KH in dKH x 10 to the power of (7 minus pH). With 4 dKH and pH 6.6, that’s about 30 ppm. It only works well with inert substrate and no peat or heavy driftwood.
What pH drop do I need for 30 ppm CO2?
About 1.0. Compare your degassed tank water with the tank during the CO2 period. A drop of 1.0 means roughly ten times the CO2 of degassed water, which is around 30 ppm.
What color should my drop checker be?
Green with 4 dKH reference fluid. Blue means too little, lime or yellow means too much. Wait two hours after any change before reading it.
Is 40 ppm CO2 too much for fish?
It can be. Some tanks run that high with healthy fish, but many fish start breathing hard above 35 to 40 ppm. Watch behavior closely and keep good surface movement.
Why is my CO2 calculator reading so high?
Most likely aquarium soil, peat or driftwood lowering your pH, which fools the KH and pH chart. Switch to the pH drop method for a true reading.