The CO2 diffuser bubble rate guide calculates a starting target bubble count per second (BPS) for your planted aquarium based on tank volume and plant density. Bubble rate is a starting point — always calibrate against pH monitoring to achieve the ideal 20–30 ppm CO2 during the photoperiod.
Tank Settings
pH Monitoring Guide
Bubble rate is a starting point. Calibrate using pH:
CO2 Schedule Recommendation
KH / pH CO2 Reference Table
Target pH during photoperiod for ~25 ppm CO2 by KH level:
| KH (°dH) | Target pH (25 ppm CO2) | Lights-off pH (typ) |
|---|---|---|
| 1 | 6.2 | 6.7 |
| 2 | 6.5 | 7.0 |
| 3 | 6.7 | 7.2 |
| 4 | 6.8 | 7.3 |
| 6 | 7.0 | 7.5 |
| 8 | 7.2 | 7.7 |
How to Set the Right CO2 Bubble Rate
CO2 injection is the single biggest limiting factor in high-tech planted tanks. Getting the bubble rate right prevents two failure modes: algae from insufficient CO2 and dead fish from CO2 excess. The BPS calculation gives you a starting point; pH monitoring gives you the ground truth.
Step 1: Start With the Calculated BPS
Use the calculator above to get a baseline. Set your regulator to approximately this bubble rate using a bubble counter on the diffuser output. Let the tank run for 3–4 days before judging the result.
Step 2: Monitor pH Swing
Measure pH one hour before lights on (lowest CO2, highest pH) and one hour before lights off (highest CO2, lowest pH). The difference should be 0.5–1.0 pH units. This method is independent of KH and works for any water chemistry.
Step 3: Watch the Drop Checker
A drop checker filled with 4 dKH reference water and bromothymol blue indicator should turn green during the photoperiod. Blue = too little CO2. Yellow = too much (dangerous). Green = 25–35 ppm — the sweet spot.
Step 4: Turn CO2 Off at Night
Plants stop consuming CO2 when lights turn off. Running CO2 at night drops pH unnecessarily and stresses fish. Use a timer to stop CO2 injection 30–60 minutes before lights off and restart it 1–2 hours before lights on.
FAQ
What is the right CO2 bubble rate for a planted tank?
A general starting point is 1 bubble per second per 50 liters for low-density planted tanks. Medium-density tanks (50-70% plant cover) need about 2 BPS per 50L. High-density tanks and demanding plants need 3+ BPS per 50L. These are starting points — adjust based on pH monitoring.
How do I know if my CO2 is too high or too low?
Monitor pH during the photoperiod vs. lights-off. CO2 should drop your pH by 0.5–1.0 units compared to your lights-off pH. A drop greater than 1.5 units risks stressing or killing fish. A drop less than 0.3 units means plants aren't getting enough CO2.
What is the ideal CO2 concentration for planted tanks?
Most planted tank guides recommend 20–30 ppm CO2 during the photoperiod. This corresponds to a specific pH based on your KH (carbonate hardness). Use the KH/pH/CO2 table to find your target pH given your KH.
When should I turn CO2 on and off?
Turn CO2 on 1–2 hours before the lights to pre-charge the water. Turn it off 1–2 hours before lights off — plants stop consuming CO2 at night and don't need it. CO2 running at night drops pH unnecessarily and wastes gas.
Is this calculator free?
Yes, completely free with no signup required.
Does bubble rate depend on the diffuser type?
Yes. Inline diffusers and reactors dissolve CO2 more efficiently than standard ceramic disc diffusers, so the same BPS delivers more dissolved CO2. If you switch diffuser types, recalibrate using pH monitoring rather than relying on a fixed BPS number.