Why does my pH drop so much overnight?

Plants consume CO2 during daylight hours (photosynthesis), raising pH. At night, plants respire and release CO2, causing pH to drop — often by 0.5–1.0 units. This is normal in a planted tank. Always measure pH at a consistent time of day (usually in the afternoon, a few hours after lights-on) to get comparable readings.

Can I use airline tubing to inject CO2?

Airline tubing and standard air check valves are not rated for pressurized CO2 systems. For pressurized CO2, always use CO2-rated tubing (more rigid, typically green or black) and a proper CO2-rated check valve. Regular airline tubing will soften and degrade under CO2 pressure over time.

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Aquarium CO2 Calculator

Calculate the ideal CO2 concentration for your planted aquarium using pH and KH. The key to lush, healthy aquatic plants.

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Carbonate Hardness (KH)(dKH)

Plant Density

CO₂ Target by Tank Type

Setup Type	CO₂ Target	Best Plant Types	Injection Method
Low Tech / Low Light	5–15 ppm	Java fern, anubias, mosses	None (relying on surface agitation & fish respiration)
Medium Tech	15–25 ppm	Stem plants, crypts, swords	DIY CO2 or small pressurized system
High Tech / High Light	25–35 ppm	HC, Monte Carlo, glosso, foreground carpets	Pressurized CO2 with diffuser & solenoid
⚠️ Danger Zone	40+ ppm	N/A — fish at risk	Reduce injection immediately; add surface agitation

CO₂ & the Day/Night Cycle

Always turn off CO2 injection 1 hour before lights out. At night, plants stop consuming CO2 and switch to respiration (releasing CO2). Continued injection overnight can cause CO2 to accumulate to dangerous levels. Use a solenoid valve on a timer to automate this. Measure pH in the afternoon (a few hours after lights-on) for the most useful, consistent readings.

The CO2 / pH / KH Triangle

In freshwater aquariums, CO2, pH, and KH (carbonate hardness) are directly linked. When CO2 dissolves in water, it forms carbonic acid, which lowers pH. The KH acts as a buffer that resists pH changes. By measuring two of the three values (pH and KH), we can calculate the third (CO2 concentration) using the formula: CO2 = 3 × KH × 10^(7-pH).

Target CO2 for Planted Tanks

For a low-tech planted tank with slow-growing plants (java fern, anubias), 5–15 ppm CO2 is sufficient. Medium-tech tanks aim for 15–25 ppm. High-tech tanks with demanding foreground carpeting plants (dwarf baby tears, Monte Carlo) need 25–35 ppm. Never exceed 40-45 ppm as this begins to suffocate fish. The optimal pH to target for a 30 ppm CO2 level depends entirely on your KH.

Frequently Asked Questions

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