Beyond the Basics of CO2: Using a Compressed Gas System.

I have a 92-gallon corner tank with a wet/dry filter and have had the tank

Q. I have a 92-gallon corner aquarium with a wet/dry filter and have had the aquarium established for about two years now with a fine grade gravel. My fish load is full, but I do not have an overloaded, freshwater fish community aquarium. I have been leaning more toward a well-planted aquarium with about 60 fish (tetras, plecos and maybe a nice trio of swords). I want to switch to a sand substrate or black onyx substrate, but have not been able to obtain much advice of pros and cons. I have read articles that say to use a small layer of peat on the bottom, and some say not to. I also want to set up a carbon dioxide injection system. I have done the do-it-yourself carbon dioxide method for a while now with small results on an aquarium this big, but it works nicely, as I haven’t overdosed the aquarium with the DIY method. I want to do it right, so I’m not worried as much about cost as about nuking a couple hundred dollars of plants and fish. If you could help me out with your opinion with the substrate and basics of carbon dioxide injection systems with PH controllers, it would be cool, too!

A. Some sand is too finely grained, and it becomes compacted too easily. If you use sand, try to find some that is about 3 mm in size. Onyx sand is a good example for size. Also, it is very porous, and like other very porous material, works well in a planted aquarium. There are several commercially available planted aquarium substrates on the market, and you can use any of them successfully. You don’t need to add any peat if your aquarium is already set up and established. The value in adding just enough sphagnum peat moss to cover the bottom glass is providing some organic matter in the substrate in a newly set-up aquarium. Alternatively, you can add some of the accumulated mulm from an established aquarium (the mulm is the gunk you vacuum off  a mature substrate or collect from a well used filter). If you add some peat and mix some mulm into a new substrate, the peat (although unnecessary) will not hurt anything.


It would be hard to hurt your fish with carbon dioxide (CO2) using a yeast fermentation bottle. In fact, you might need a couple of bottles to maintain good CO2 levels in a 92-gallon aquarium. Aim for about 20 parts per million (ppm) of carbon dioxide.

The yeast method is inexpensive to set up but requires more maintenance than using a compressed gas tank of CO2. Carbon dioxide in the water will cause stress to fish, unless the CO2 level gets up well past 40 or 50 ppm.

If you use the compressed gas method, these are the things you will need:

  • A compressed gas tank with a CGA-320 valve
  • A gas pressure regulator made to fit a CGA-320 tank valve
  • A needle or metering valve
  • Some airline tubing
  • An injector, mixing chamber, super-fine airstone, powerhead or canister filter

“CGA” stands for “Compressed Gas Association,” which is the recognized standard organization for compressed gas. “CGA-320” is the standard for compressed carbon dioxide equipment. Any place that sells compressed gas tanks will be familiar with the CGA numbering system and which CGA-xxx goes with what gas. Some fish stores, especially Internet stores, sell compressed gas tanks, as do welding supply stores. These stores also sell gas pressure regulators. The regulator maintains a steady usable low pressure on the gas line. You can pay anywhere from $30 to hundreds of dollars for a regulator. For aquatic gardening purposes, the inexpensive ones are adequate.

If the regulator regulates the gas pressure on the working gas line, why is another valve needed? The regulator converts the high pressure of the gas inside the CO2 tank (about 600 to 900 psi) to about 10 to 30 psi. However, it does not limit the flow rate of gas through the gas line. A needle valve, also called a “metering valve,” is a small valve that limits the rate of flow. This is important for two reasons. First, it allows you to adjust the gas flow rate with more precision than you can with a regulator. Second (and very important), it limits the maximum flow rate so that once set correctly, carbon dioxide cannot flow too quickly through the system. Stores that sell CO2 equipment for planted aquariums sell needle valves for as low as about $18.

A perhaps ironic fact about gas pressure regulators is that when the gas tank gets near empty, the low side pressure goes up. The pressure in the gas tank will remain the same until it is nearly empty, then the pressure begins to drop as more gas leaves the tank. When the tank pressure drops, the regulator cannot regulate the output pressure as well, and the low pressure on the output side of the valve increases. This could result in an increased flow rate of CO2, which could put too much carbon dioxide into your aquarium too quickly. The needle valve prevents that from happening. To do so, it is critical that you adjust the regulator so that the low pressure is high enough that the needle valve is limiting/controlling the rate of flow.

Does this sound a bit scary for the fish? It is actually not a big problem. When a CO2 tank is filled/refilled, most of the carbon dioxide is compressed into a liquid state inside the tank, and only the upper part of the tank contains CO2 in gas form. As CO2 gas is released from the top of the tank, more of the liquid carbon dioxide evaporates to gas inside the tank, and the tank pressure remains constant. Eventually, there is no more liquid CO2 in the tank. At that point, as carbon dioxide continues to be released from the tank, the pressure in the tank finally starts to drop. However, when there is no more liquid left in the tank, there is enough gas to keep running your system for days, if not weeks. How long it continues depends on how much CO2 your aquarium uses. Several days or weeks is plenty of time for you to notice that it is time to refill the CO2 tank. A good rule of thumb is to plan the next refill when you see the high pressure dropping and to get the refill before it drops all the way down to about 400 to 500 psi. That should leave you a very good margin of error and avoid significant changes in the flow rate. Note that of all the CO2 that was in the CO2 tank to begin with, the proportion that is in the tank when all the liquid CO2 is evaporated is very small – just pennies’ worth. Do not worry about using the very last cent of carbon dioxide. When the pressure drops, get a refill.

You need to check the regulator high-pressure gauge every now and then to see if the high pressure is dropping. That might sound like a lot of work, but it is a lot less work than refreshing a yeast bottle every month of so. How often you will need to refill a CO2 tank depends on the size of the CO2 tank, and how quickly your aquarium uses and sheds carbon dioxide. On my 150-gallon aquarium, which has a wet/dry filter and sheds CO2 very quickly, a 10-pound CO2 tank lasts about two months. I check the gauge about once a week. On another aquarium that is smaller and set up to not shed CO2 gas quickly, a 5-pound tank lasts about 18 months. I check that gauge about once a week just to keep up the habit, but once a month would work, too.

Refilling a larger CO2 tank often costs about the same as refilling a smaller one. For example, I pay $9.50 for a 5-pound refill and $10 for 10-pound refill. Therefore, buying a larger CO2 tank can be cheaper in the long run. Get the largest CO2 tank that you can easily handle and that fits the location you intend for it’s use.

You can feed the carbon dioxide into the aquarium using the same method you use for a yeast bottle. You can feed it into the intake of a canister filter or powerhead or use a super fine airstone. Carbon dioxide is more efficiently absorbed into the water with a mixing chamber (sometimes called by the excessively technical term “CO2 reactor”). You can find these at stores that sell CO2 equipment for planted aquaria, or you can easily and cheaply make your own from PVC pipe and parts.

To set the flow rate initially, start out with about 20 psi on the regulator, and adjust the needle valve for a flow of about 30 bubbles per minute. That is just a start up – do not rely on bubble rates for assessing your CO2 level in the water. You can check the flow rate by temporarily holding the end of the gas line underwater. Measure the carbon dioxide level in the water after about six hours, and adjust the flow rate. Repeat measurement and adjustments as necessary. You might have to raise or lower the regulator pressure, as well as the needle valve, going back and forth between the two until you get a stable setting. Once you get things set, you probably will not have to fiddle with the settings for months.

I offer some last comments on equipment. You do not need a bubble counter, pH controller or solenoid. If you run the carbon dioxide at a constant steady rate 24 hours per day, seven days per week, you will have a relatively steady carbon dioxide level. During the day, the level will decline as the plants photosynthesize, and it will rise again overnight. Nevertheless, the change is small, as evidenced by the change in pH over the course of 24 hours, which is negligible (about 0.2 units). If you want to save a little on CO2, you can attach a solenoid between the regulator and the needle valve, and plug the solenoid into the aquarium light timer. Then the CO2 will flow only when the aquarium plants photosynthesize. Because you are not slowly building up the CO2 level overnight from the depleted level of the day before, you will need to run the CO2 at a slightly faster flow rate to get the carbon dioxide level up quickly once the lights turn on. Still, you can expect a pH swing of only about 0.2 units. If you want to spend more money, you can plug the solenoid into a pH controller. You can get a reasonably priced pH controller that works reliably for about $80 or so, plus about $50 per year for annual replacement of the probe. A pH controller turns the carbon dioxide flow on/off based on small changes in the pH. This keeps the CO2 level stable as evidenced by the pH level that will vary within about 0.2 units. Your plants will not care which of the three “control” methods you use: 24/7 operation, light timer or pH controller. The plants will just love the carbon dioxide, and your fish will be safe and happy in your flourishing aquatic garden.

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