A Primer on Passing Gas

By gas, we obviously mean CO2! Plants, whether aquatic or not require carbon dioxide. The scientific shorthand way of writing carbon dioxide is C (one part carbon), O2 (two parts oxygen) = CO2. Carbon dioxide is a natural product of respiration. The air we inhale (and fish breathe) contains a portion of oxygen (around 21%), and when we breathe out, the mix of gasses we exhale has shifted to contain slightly less oxygen and slightly more CO2. This works in balance with organisms like plants which utilize that CO2 to produce sugar and oxygen in the presence of light in a process called photosynthesis.

While our fishy friends don't have lungs, they live in the same balance with aquatic plants. Fish remove oxygen from water while “breathing” with their gills and add to the balance of carbon dioxide. The aquatic plants then utilize that carbon dioxide the same way their terrestrial counterparts do.

The balance of available oxygen and carbon dioxide in aquatic environments is also affected by some natural laws, the first being equilibrium. Equilibrium means that the natural state of things is to move from areas of high concentration to areas of low concentration. If there is more carbon dioxide in air than in the water, the water will “absorb” more CO2 until balance is reached. Another important law impacting this is called Boyle's law, which loosely translated says that cold liquids hold gas better than warm liquids. This is why a cold can of soda stays bubbly longer than a warm one (there is a pressure element here too, the gas can be pressurized to squeeze into liquid better – this doesn't really apply to aquariums until we get into supplementing CO2.)

Why are we telling you this? Well because you've been asking about supplementing carbon dioxide. If you only have fish, and no interest in keeping plants, you can stop reading now and try one of the hundreds of other articles on the site! If you're thinking about keeping plants, read on!

This natural state of balance (equilibrium again) between atmospheric (the air we're breathing) CO2 and the CO2 in your aquarium means your aquarium will have a concentration of somewhere between 2ppm and 5ppm carbon dioxide. As the volume of air outside the tank is massively bigger than the volume of water inside it, the room will keep absorbing carbon dioxide from the aquarium no matter how much you add; conversely, no matter how much oxygen you add, the water can only contain a certain concentration of that as well before the balance reaches the saturation point and some of the oxygen is lost to the air in the room. The balance of oxygen in your tank will compete with the balance of carbon dioxide too. The saturation point for all gasses in your system is cumulative, if you force oxygen in, it will have to drive some other gasses out. (This is why oxygen stones will help you raise the pH in your aquarium).

There is a good number of nice plants available to you through Aquatropic's store partners that don't require supplemental CO2 and we've covered these in other articles. They are often referred to as low-tech plants, or plants suitable for low-tech aquariums. The phrase “low-tech” really only means aquariums that don't have supplemental CO2 because the systems that deliver carbon dioxide add some technical complexity that tanks without plants don't need. However, there are many plants that just will not thrive unless the balance of fertilizer, light and CO2 is just right. Truthfully, even the low-tech plants will do better with supplemental carbon dioxide.

So how do you know when your aquarium is in need of supplementing? Well, if you've added plants, those plants are not thriving, then they are asking for something. That something could be light, or plant food, or carbon dioxide. If your plants are growling slowly, if the plants are turning brown or getting brown spots, if your plants are not producing little oxygen bubbles on their surface during daylight hours or just failing to thrive, a lack of CO2 is a likely culprit.

In a captive aquatic environment, plants will do best with CO2 levels that are in the 10ppm to 30ppm range, and because of the reasons we've discussed, this is only possible with supplementation. There are a few ways to do this, but generally they all boil down to a carbon dioxide source, control of the flow of the gas (on / off / flow rate), and a mechanism for diffusing the gas in your aquarium.

The sources for CO2 are usually canisters of varying size. This is good as these canisters are used for a variety of other reasons and are pretty easily available. Some will be system specific, and other (generally larger and cheaper ones) will have other uses like carbonating beer for the tap at your local pub. There are also systems where you can mix a couple dry compounds to produce CO2 not unlike the volcano science experiment you likely did in grade school.

There are also a wide variety of control mechanisms and diffusers, and for these it will be easiest to use aquarium specific ones. Diffusers can be plates that you run bubbles through in an aquarium, or they can be plumbed in line to diffuse gas into the returns of your aquarium filter plumbing. The flow rate control is important because the plants we're trying to successfully grow are dependent not just on copious amounts of CO2, but instead a balance of gas with light and nutrients. Too much CO2 with not enough light or not enough nutrient will cause your plants (and fish) to suffer as well.

It is for this reason that we suggest using substrate designed to planted tanks to supply nutrients, and to start with short days and low supplementation levels. Start by having lights that are set for a six-to-eight-hour day length and a low-level supplementation. Follow the instructions on whatever supplemental gas systems you choose to go with. Make adjustments with light and gas slowly over time. Be patient. Get the balance right between these elements before you start adding fish and inverts.

The fish and inverts you add will also bring up the CO2 level naturally, but unless your stocking density is very high, their impact on gas levels will be nominal and you should be fairly easily able to compensate for their addition. Adding these fish later will prevent them from having to live through the fine tuning that all beautiful, planted tanks will require.

We've presented this to you as a primer on gas in aquariums. If you're a hobbyist who likes to tinker with their aquariums, a high-tech planted tank should be on your horizon. A well-executed planted tank rivals any other aquarium for beauty including the reef tanks popular on the saltwater side of the hobby. There is plenty of nuance to dosing gas, and a huge variety of videos and tutorials on YouTube for you to get lost in. Over the next few weeks, we'll get into some details on gas, light and tuning these tanks for you to get the displays you've been wanting to achieve. For now, head over to your Local Fish Store, and ask some questions. Tell them Aquatropic sent you.