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How much CO2 should I inject?

Updated: Feb 2

So you've purchased all your CO2 equipment, but now what? We need to dial in your CO2, because too little CO2 can cause algae, but too much can kill livestock.

Drop checkers and bubble counters are just too inaccurate to measure the volume of Co2 we are injecting. There is no standard bubble size as all bubble counters are different, so we can't just say, inject 3 bubbles per second. Also, when you get to really larger tanks, you're injecting so much that you can't even see how many bubbles are being injected. And with drop checkers, the solution in the drop checker overtime needs to be replaced leaving you inaccurate readings over time. And they also have a 1-2 hour lag, so by the time your fish are at the top gasping for air, it might be too late. Now I don't think they are useless as I kind of like bubble counters as a visual indication the gas is flowing on smaller tanks, and a drop checker can be good visual feedback on a larger tank, but I would not use them as a way to dial in CO2.

The PH drop method

A more accurate way would be using the PH drop method. We know that when injecting CO2 your PH is going to drop. We also know that around 30ppm of CO2 injected into our aquariums, so the sweet spot is injecting enough to drop your PH a full point prior to your lights coming on! It's just that simple. So how do we get this done? Let's look at the example below:

Step 1: Measure your PH with absolutely no CO2 injected into the water column. This can take up to 2 days with good surface agitation if you've already been injecting. For example, sake, let's say our PH reading is 7.5.

Step 2: Have your CO2 injection turn on 1 hour before your lights come on.

Step 3: Start injecting and test the PH right before the lights come on, which should be one hour later. If your PH is 6.5, then you've achieved the ideal injection rate. If you're PH is anywhere between 7.5, and 6.6, then you need to inject a little more.

Some things to look out for

There are 5 variables we are dealing with.

1) Rate of injection - I suggest not trying to balance everything all at once by keeping your lead-in time the same (1 hour) and just messing with your rate of injection. There is a point where you'll max out your diffuser by injecting too much and the bubbles get so big they just fly up to the water's surface and pop out the top. This is where you'll either need to purchase a larger regulator or increase the lead in time to 1.25-1.5 hours before the lights come on.

2) Lead in time - If you're maxing out your diffuser and don't want to get another one, increase the amount of time the CO2 is on before the lights turn on.

3) Method of diffusion - This is extremely important as not all methods of diffusion are equal. For tanks 10-30 gallons, most people can get away with a little ceramic diffuser in the tank. This a cheap and easy way to start out with CO2 injection. However, larger tanks, like 75-125+ need a lot more CO2, so an inline atomizer is preferable for them as there is almost 100% diffusion (bubbles are not escaping out the top of the tank). Again, we don't want to turn up our regulators so high that the bubbles just fly out of the tank. We want a really fine mist to swirl around our tank preferably making contact with the plants.

4) Surface agitation - We need to be aware of surface agitation as too much will off-gas the CO2 we are injecting. Just know that if you have a reef tank with a sump below, you'll probably need to inject more CO2 than someone without all that agitation. The same off-gassing effect happens when someone runs an airstone... not because they are injecting air, but because the surface agitation releases the unnatural CO2 levels we are keeping in the tank.

5) Your KH (alkalinity) hardness - Tanks with a high KH will need to inject more CO2. The higher the KH, the more stubborn your PH will lowering itself.

And a final thing to note, the more plant mass you have, the CO2 you'll need throughout the day. This will be a little bit of a moving target. As your plants mature and fill in, we'll most likely need to increase the rate of injection to keep up with their demand. Inversely, if you do a massive trimming, you'll probably need to turn down the CO2 a smidge till the plants grow back. So it's not really a set it and forget it. If you don't want to hassle with this at all, look into a PH controller like this one. PH controllers are totally unnecessary for most tanks and are just a way to automate things.

If you are more of a visual learner, this video talks about the PH drop method:

Shoot, I injected too much

If you see your fish gasping for air at the top, immediately toss in an air stone and pump oxygen into the tank. As explained, surface agitation helps degas the water, so if you have a canister filter, you can also point your inlet at the water surface to create more agitation. It doesn't take much time, but you should see your fish returning to normal activity within 15-30 minutes.

Uuuum, clean up on isle 9, my ceramic diffuser is clogged

Cleaning a diffuser is super easy. Toss into a mug, 1 part household bleach, 3 parts warm water. With the CO2 on, toss the diffuser into the mug and let it sit for 10-15 minutes, depending on how much algae you have. After the diffuser is clean, rinse out the mug REALLY well and fill it up with 1 part de-chlorinator, and 3 parts water and let that sit for another 10 minutes. This will ensure that all the bleach has been neutralized and you're good to drop it back into your tank! It's important to clean the diffuser while the CO2 is on. This will prevent the bleach from getting into the diffuser and CO2 tubing.

We hope this article helps and share it with others looking to dial in their CO2!

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