7 Reasons Why Your Aquarium CO2 is Failing (& How to Fix It!)

Struggling with CO2 in your planted tank? Here are 7 common issues and how to fix them for lush, healthy plants!

1. No Bubbles Coming through the Diffuser

If you're not seeing any bubbles coming through your CO2 diffuser, try turning up the pressure on your CO2 cylinder to push out any trapped air. Once you start seeing bubbles, be sure to turn the pressure back down to safe levels.

If you're still having issues, it's time to check for potential leaks in your system. Check the black O-ring under the bubble counter to ensure it's not cracked or missing, all your fittings to ensure they are tight, and your tubing to make sure there's no cracks or holes. If this doesn't solve the problem, check out issue #2!

2. Check Valve

If you've accidentally installed your check valve upside-down. Check valves are meant to ensure the air only flows in one direction, which protects your solenoid from water backflow. There should be an arrow marking the correct direction for air flow on your check valve-- check to ensure that it has been installed correctly. If it's reversed, then you won't be getting any air into your diffuser!

3. Wrong Tubing for CO2 Gas

CO2 setups require special tubing! You may be using tubing that is for air pumps or other uses, but may not be suitable for high pressurized setups like CO2 injection. CO2 requires polyurethane tubing, which will prevent gas from leaking from your system.

4. Wrong Diffuser Type, Size or Placement

Nano diffusers should be used for nano tanks, and large diffusers should be used for large tanks. The size differences help to support the level of CO2 needed for your particular tank size, which helps to avoid underdosing or overdosing.

If you place the diffuser too high in your tank, you'll end up with too much CO2 escaping out the top instead of dissolving into the water column, so slide it down to the bottom of your tank instead.

5. Bubble Counter Not Working

If you don't have your bubble counter calibrated properly, it could lead to CO2 failures. For tanks in the 5-10 gallon range, we recommend about a bubble per second. For 15-40 gallons, 2-3 bubbles per second. For tanks 50 gallons or more, aim for 3-5 bubbles per second.

One way to check that you're releasing the correct amount of CO2 is to use a drop checker. If the pH indicator solution is green, then you're dosing healthy levels. If the solution is yellow, then you are injecting too much CO2. If it's blue, then you don't have enough CO2.

Another issue you could face with the bubble counter is the water evaporating out completely. To avoid quick evaporation, we suggest filling it up with mineral oil instead.

6. Wrong CO2 Schedule

CO2 & lighting are intrinsically related. When your light is on, your plants are absorbing CO2 and releasing oxygen. When the lights are off, the plants stop this process. Running your CO2 while your lights are off can lead to too high CO2 levels and too low O2 levels in your tank, putting your fish's health and safety at risk.

You can solve this issue by using a timer to control when your CO2 solenoid powers on and off. We recommend having the CO2 kick on about an hour before your lights so that the CO2 has time to build up in the water column to be absorbed by your plants once the light comes on. Similarly, you should turn off the CO2 about 30 minutes after your lights go off to ensure too much CO2 doesn't build up in the system.

7. CO2 Tank Dump

Everyone fears the dreaded CO2 dump-- when there's a rapid loss of pressure in your CO2 system that can overdose CO2 into your tank and harm your fish. This can happen when the level in the CO2 cylinder gets so low that a single stage regulator can no longer control it, so it dumps into your tank unregulated. You can solve this by using a Dual Stage regulator rather than a Single Stage one. This will control the pressure at two different points from the cylinder into the regulator and from the regulator out into your aquarium.