Aquarium CO2 Injection Calculator
Created by: Emma Collins
Last updated:
Calculate optimal CO2 injection rates for your planted aquarium. Get bubble rate recommendations based on tank size and plant demand, estimate how long your CO2 cylinder will last, and calculate current CO2 levels from pH and KH water test readings.
Aquarium CO2 Injection Calculator
AquariumCalculate bubble rates, CO2 levels, and cylinder duration for planted tanks
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What is an Aquarium CO2 Injection Calculator?
An Aquarium CO2 Injection Calculator helps planted tank enthusiasts determine optimal CO2 injection rates, estimate cylinder duration, and calculate CO2 levels from pH and KH readings. Carbon dioxide supplementation is essential for growing demanding aquatic plants and achieving lush, vibrant planted aquariums.
This calculator takes the guesswork out of CO2 injection by providing bubble rate estimates based on tank volume, calculating how long your CO2 cylinder will last, and helping you interpret drop checker colors and pH/KH relationships.
Whether you're setting up your first pressurized CO2 system or optimizing an existing setup, this tool provides the calculations needed for healthy plant growth without endangering fish.
CO2 Calculation Formulas
CO2 from pH and KH:
CO2 (ppm) = 3 × KH (dKH) × 10^(7 - pH)
Bubble Rate Estimate:
Bubbles per second = Tank Gallons × 0.1 × Plant Demand Factor
Cylinder Duration:
Days = (Cylinder oz × 28.35) ÷ (Daily CO2 grams)
Daily CO2 ≈ Bubble Rate × Hours × 0.002 grams/bubble
Target Levels:
• Low-tech plants: 10-20 ppm
• Medium demand: 20-30 ppm
• High demand: 30-40 ppm
CO2 Calculation Examples
Example 1: Estimating CO2 from pH/KH
KH: 4 dKH, pH: 6.8
CO2 = 3 × 4 × 10^(7-6.8) = 3 × 4 × 1.58 = 19 ppm
Result: Slightly low, increase injection rate
Example 2: Bubble Rate for 29-Gallon Planted Tank
High plant demand, target 30 ppm
Base rate: 29 × 0.1 = 2.9 BPS
With high demand factor (1.3): ~3.8 BPS
Start at 3-4 bubbles per second
Example 3: 5-lb Cylinder Duration
Tank: 55 gallons, 3 BPS, 8 hours/day
Daily usage: 3 × 8 × 3600 × 0.002 = 173g/day
Cylinder: 5 lb = 2268g
Duration: 2268 ÷ 173 = ~13 weeks
CO2 System Applications
High-Tech Planted Tanks
Demanding plants like HC Cuba, Monte Carlo, and red plants require consistent CO2 at 30+ ppm. Use pressurized systems with solenoids for reliable, automated injection.
Low-Tech Plant Boosting
Even "low-tech" tanks benefit from moderate CO2 supplementation. DIY systems or low injection rates (1-2 BPS) can significantly improve growth of easy plants.
Aquascaping Competition Tanks
Competition aquascapes require precise CO2 control for optimal coloration and dense growth. Calculate exact requirements to balance plant health and fish safety.
Budget CO2 Planning
Calculate cylinder usage to plan refill costs. Larger cylinders cost less per ounce - a 20-lb cylinder may save money over multiple 5-lb refills.
Tips for CO2 Injection Success
Start Low, Go Slow: Begin at half the calculated rate and increase gradually over 1-2 weeks while monitoring fish behavior and drop checker color.
Use a Timer: Run CO2 only during photoperiod. Turn on 1 hour before lights, off 1 hour before lights off. This maximizes plant uptake and minimizes waste.
Maintain Stable KH: Fluctuating KH causes pH swings that stress fish. Keep KH stable at 3-5 dKH for predictable pH response to CO2 injection.
Check Drop Checker Regularly: Drop checkers lag 1-2 hours behind actual levels. Check mid-photoperiod for accurate readings. Replace solution monthly.
Frequently Asked Questions
How much CO2 do planted aquariums need?
Most planted tanks thrive at 20-30 ppm CO2. Low-tech plants survive at 5-15 ppm (natural levels), while demanding plants like carpeting species need 25-35 ppm. Use a drop checker with 4dKH solution to monitor levels - green indicates ~30 ppm.
How many bubbles per second should my CO2 be?
Start with 1 bubble per second per 10 gallons as a baseline. A 30-gallon tank might need 3 bubbles per second. Adjust based on drop checker color - increase if blue, decrease if yellow. Every tank is different based on surface agitation and plant mass.
How long does a CO2 cylinder last in an aquarium?
A 5-lb CO2 cylinder typically lasts 2-6 months depending on tank size and injection rate. Larger tanks or higher bubble rates deplete cylinders faster. A 20-gallon tank at moderate injection might get 4-6 months from a 5-lb cylinder.
Should I run CO2 at night in my aquarium?
No, turn CO2 off at night or 1 hour before lights off. Plants only use CO2 during photosynthesis (when lights are on). Running CO2 at night wastes gas and can dangerously lower pH while building up excess CO2 that stresses fish.
What color should my CO2 drop checker be?
Green indicates approximately 30 ppm - ideal for most planted tanks. Blue means CO2 is too low (under 20 ppm). Yellow means CO2 is too high (over 40 ppm) and potentially dangerous for fish. Check drop checker 2-3 hours after lights on.
Can too much CO2 kill fish?
Yes, CO2 levels above 40-50 ppm stress fish, and very high levels (100+ ppm) can be fatal. Signs of excess CO2 include fish gasping at surface, rapid gill movement, and lethargy. If fish show distress, increase surface agitation immediately to off-gas CO2.
What's the difference between pressurized CO2 and DIY CO2?
Pressurized CO2 (cylinders with regulators) provides consistent, adjustable output and is more reliable. DIY CO2 (yeast/sugar or citric acid) is cheaper but inconsistent, harder to control, and requires frequent maintenance. Pressurized is recommended for serious planted tanks.
How do I calculate CO2 from pH and KH?
CO2 (ppm) can be estimated from pH and KH using the formula: CO2 = 3 × KH × 10^(7-pH). For example, KH 4 and pH 6.8 gives approximately 30 ppm CO2. This relationship is why stable KH is important for consistent CO2 levels.
Sources and References
- Aquatic Gardeners Association, "CO2 in the Planted Aquarium", AGA, 2024
- The Planted Tank Forum, "CO2 Injection Best Practices", TPT, 2024
- Tropica Aquarium Plants, "CO2 and Plant Growth Guide", Tropica, 2024