With the risk of oversimplification, a good starting target would be attaining CO2 in the region of 20ppm -35ppm. If using a bubble counter – start at one bubble per second per 20 gallons of tank volume, then slowly tune upwards while measuring the pH drop. (It can take a few hours for the pH to drop fully and stabilize)

The way to gauge this is through measuring the change in pH/KH as a proxy for the amount of CO2 dissolved, as described in more detail here. This does require the ability to test pH and KH which is a necessary investment for newer aquarists. Generally, if your KH is between 1 to 10, aiming for a 1 point drop in pH (from when CO2 injection is off to peak saturation) will put you at good levels.

If livestock is already present while CO2 is being adjusted, livestock must be observed closely for signs of excessive CO2 saturation. There can be a significant lag effect between CO2 adjustment and the time it takes for water to be saturated.

Using a drop-checker takes almost the same amount of effort, with far less consistent results- read further for the explanation.

I would use an automatic timer to begin CO2 injection at least an hour, if not 2 hours before the tank lights turn on. Plants draw CO2 most strongly in the initial hours of light exposure, so having good CO2 levels upon lights on is ideal.

One can turn CO2 injection off an hour before the lights off. This does depend on how short the light cycle is; if the light cycle is 6 hours or shorter, I would recommend running CO2 for the entire light period.

For my tanks, lights and CO2 injection are turned on for a duration of 8 hours per day.

For beginners afraid of algae, using a short light cycle such as 6 hours can be helpful. For stable, matured tanks, light duration can be pushed to 10hours+ without adverse effects.

If you are installing your CO2 system for the first time (or for a new tank), I suggest the following steps:

After attaching the regulator to the cylinder and setting up all necessary tubing, I would check for leaks using soapy water; using a paintbrush, paint soapy water over all joints in the system. If there is a leak, large soap bubbles will be visible.

Next I would take the pH/KH of the tank water before CO2 injection starts. This is the baseline pH/KH value. As CO2 injection starts, the pH of the tank water will drop as a portion of the injected CO2 is converted to carbonic acid.

Using the bubble counter, start injection at 1 bubble per 20 gallons of tank water. Bubble counters have different bubble sizes and this is just an extremely rough gauge as a start point. The bubble count used may eventually be tuned much much higher.

When releasing livestock into the tank, always release them during the period that CO2 injection is turned off. When levels build up gradually during the next CO2 cycle they will have some time to adapt.

As CO2 is dissolved in water, a portion of it turns into carbonic acid, lowering the pH. CO2 levels can thus be gauged by testing how far the pH in the tank decreases when CO2 is being injected. This is done by by exploiting the ph:KH relationship – for a known KH value, the pH drops at a predictable rate as CO2 is being injected into the tank. To read more in-depth on how to do this accurately head here.

Many hobbyists like using drop checkers but they are not a reliable method of determining CO2 levels (see below). To read more on how to do it accurately, head to this section.

You should be present to observe planted tank lifestock for distress anytime CO2 is tuned up. CO2 build-up can take hours, so plan ahead accordingly.

For those who exercise at the gym, this is like using a guided bench press rack: it allows overload (in this case CO2) while providing a safety net.

Off-gassing occurs whenever there is contact time and circulation between the water surface and deeper water layers in the planted aquarium. It brings tank O2 & CO2 levels closer to atmospheric concentrations. In our planted tanks this means that it off-gases CO2 while increasing O2 levels. This seems counter-intuitive at first, but it makes tuning CO2 levels high much easier. To read exactly why this is so, head to this section.

To have good gaseous exchange, have a decent amount of surface agitation, a clean water surface and/or use sump/overflow systems.

​I personally use surface skimmers  on all my tanks but any device that causes circulation of surface and deeper layers of water in the tank will work.

Apart from learning to tune CO2 levels well, distributing it evenly throughout the tank is the single biggest impact factor in a CO2 injected planted tank.

If using a standard CO2 diffuser, it should be placed in the down wash of flow on the opposite wall from the filter outlet. (red D below).

CO2 mist should be carried by the current all the way to hit the yellow Y in the picture.

​If this is not achievable, it means that either the filter flow rate is too low for the tank size or that the CO2 diffuser is not producing fine enough mist.