pH-Controlled CO₂: Stable Gas Delivery for a Planted Aquarium

In a high-tech planted aquarium, CO₂ is the primary fertilizer—and simultaneously the biggest risk to your fish. The Oquari system, in its Plant profile, turns a standard pH probe and a solenoid valve into a precise regulator. It keeps CO₂ levels steady throughout the photoperiod and strictly prevents gas injection at night.

Using pH as a Proxy for CO₂

Dissolved CO₂ acidifies water: the more gas present, the lower the pH becomes. With stable carbonate hardness (KH), this relationship is predictable enough that pH acts as an indirect measure of CO₂ concentration. This is standard practice; you can find pH/KH/CO₂ reference tables in any reputable planted aquarium guide.

For example, with a KH of 4, a pH of around 6.8 corresponds to roughly 30 mg/L of CO₂—a typical target for high-tech setups. Instead of dosing gas by "feel" or counting bubbles, you set a target pH, and the controller doses exactly enough gas to maintain it.

Test your KH before configuration and after any major substrate or hardscape changes. The pH/CO₂ table is only valid for your current KH. Determine your target pH from the table for ~25–30 mg/L CO₂, then verify with a drop checker and by observing your fish.

Hardware: Probe, Valve, and Relay

• pH Probe — Connected to the Oquari-Sense module (an isolated, precise measurement circuit resistant to interference from pumps and lights). Calibrate the probe using the setup wizard before first use.
• Solenoid Valve — The Oquari-Core has a dedicated low-voltage output for CO₂ valves; alternatively, you can plug a 230 V valve into an Oquari-PWR socket. Install the valve in the gas line between the cylinder and diffuser; no power means the valve stays closed.
• Rest of the CO₂ Setup — Cylinder, regulator, fine-adjustment needle valve, bubble counter, and diffuser remain unchanged, just like any standard setup.

Adjust your fine-adjustment needle valve so that when the solenoid opens, gas flows slightly faster than the tank consumes it. This allows regulation through short, precise bursts rather than continuous flow.

Reverse Hysteresis — The Opposite of a Heater

Regulation works as a mirror image of heating. A heater turns on when it is too cold; the CO₂ valve opens when pH is too high (meaning there is too little gas). For a target pH of 6.8 and hysteresis of 0.1:

• The valve opens when pH rises above 6.85.
• The valve closes when pH drops below 6.75.
• Between these thresholds, the state remains unchanged.

Hysteresis protects the solenoid valve from rapid cycling (pH readings always have slight noise) and keeps CO₂ within a narrow, stable band. Typical hysteresis settings range from 0.05 to 0.15 pH units.

Time Window: No Gas at Night

Plants do not consume CO₂ at night—photosynthesis stops, and excess gas would only stress fish and waste cylinder content. Worse, pH naturally drops overnight, so a regulator without time limits would eagerly over-dose. Therefore, CO₂ regulation is gated by the light schedule:

• Dosing begins with a set lead time before dawn (default one hour—allowing water to saturate before photosynthesis starts) and ends before dusk (remaining gas will be consumed).
• Outside this window, the valve is held closed by a safety layer—the same overarching force that triggers emergency cut-offs for heaters. No rule or command can override it.
• Change your light plan, and the CO₂ window shifts automatically. They cannot get out of sync.

Two additional safety nets: a hard lower pH limit—if pH drops below this minimum, the valve closes and alarms regardless of regulation; and probe fail-safe—if the pH probe stops responding or reports an error, the valve closes immediately rather than dosing blindly.

Configuration in the Panel

1. Select the Plant profile (if not already done during initial setup).
2. Navigate to Panel → CO₂.
3. Set Target pH (from your pH/KH/CO₂ table based on current KH) and Hysteresis (start with 0.1).
4. Set Lead time before dawn and Stop time before dusk (default: 60 minutes each).
5. Set Hard lower pH limit—below this, the valve always closes.
6. Save settings.

Recommended Starting Settings

Make changes gradually. Lower target pH by 0.05–0.1 every few days, observing fish at dawn—this is when CO₂ concentration peaks. Gasping at the surface means: step back your settings.

Najczęstsze pytania

Why use pH instead of a direct CO₂ sensor?

Direct CO₂ sensors are expensive and require frequent calibration. Using pH with known KH is a proven, cost-effective method that provides reliable indirect measurement for most planted aquariums.

What happens if the power goes out during dosing?

The system is designed with safety in mind. If power fails, the solenoid valve closes immediately, preventing uncontrolled gas release. Upon power restoration, the system resumes normal operation based on current conditions.

Can I adjust the light schedule without breaking CO₂ timing?

No need to worry. The CO₂ window automatically follows your light schedule. Any change to lighting times shifts the dosing window accordingly, ensuring synchronization is always maintained.

How do I know if my pH probe needs recalibration?

If readings seem unstable or drift significantly from expected values, recalibrate using the built-in wizard. Regular maintenance ensures accuracy and prevents false alarms or incorrect dosing.

Is it safe for fish if CO₂ levels spike temporarily?

Yes, because of multiple safeguards: hysteresis prevents rapid changes, time windows limit exposure, and hard limits shut off gas if pH drops too low. These layers protect livestock even during unexpected fluctuations.