CO2 diffusion methods

Diffusing CO2 into the tank generally uses one of the following types of equipment:

In-tank Diffuser

The most conventional device, it can be effective if used correctly. Ideally you should aim for a fine mist. It requires much more maintenance than in-line devices due to its exposure to the tank environment (i.e. it will clog up with debris etc.) and has more weaknesses compared to other methods. If you are using an in-tank diffuser, these tips may help:

Placement is critical

Placement in the tank is important. The diffuser should be placed in the down wash flow; see picture below. Given the circulating flow pattern created by the filter outflow; the diffuser should be placed where the blue X is.

The flow should carry the bubbles from the blue to the other end of the tank. If the bubbles rise vertically from the X instead of floating down with the downward current, it means that either the diffuser is ineffective; producing bubbles that are too large, or that the amount of flow in the tank is too weak.

Key Downside

The downside of in-tank diffusers is that their efficacy relies on their cleanliness, as well as consistency of flow pattern in the tank. As the diffuser clogs, efficiency is affected. Similarly, filters can slow down as they get clogged, affecting flow. Due to most hobbyists being not super-consistent with tank cleanliness, this often results in fluctuating CO2 levels. Being in tank also means that it can be blocked by plant growth.

As mentioned, efficiency rate of CO2 diffuser in tank is affected by changes in flow pattern. For example, tanks using HOB filters where flow pattern change with the water level, or tanks where dense plant growth change the flow patterns over time will face unstable CO2 levels.

Even large sized tanks can be run using in-tank diffusers (if they are serviced regularly and flow is not obstructed).

Inline atomizer

Good ones produce a smoke-like fine mist.

Atomisers require far less frequent cleaning than in-tank equipment and are less affected by changes in the flow pattern in the tank than in-tank equipment. We see no reason to have more equipment in the tank, so we will always prefer to use inline atomisers over in-tank equipment. The main disadvantage is that the fog can be visually distracting - many people prefer clear tanks. The advantage of constant fog is that you can observe the flow patterns in the tank; whether the flow is adequately hitting all the plants.

Quality Matters

Atomisers vary in quality; some produce a very fine smoke-like mist, while those that are not so well made produce more visible bubbles. We have not seen a brand that is 100% consistent, so when buying atomisers we advise you to buy from a shop that allows returns if the atomiser produces a mist that is not as fine as you would like.

Volume workhorse

Unlike reactors, which operate within certain flow and injection rates, inline atomisers can handle huge amounts of CO2 input. A single (well designed) nebuliser will be sufficient to fully saturate a 1000 gallon (3800L~) tank at normal injection rates, provided it is channeled through a strong flow.

Easy Maintenance

Similar to diffusers, nebulisers can be cleaned by soaking in bleach or H2O2 solution. Because they are not in the tank environment, they require much less maintenance - once every 2-3 months. This is one of the main advantages of inline equipment. Some inline nebulisers are easier to disassemble, depending on their design.

Inline atomisers and inline reactors reduce the need for equipment in crowded tanks. (sometimes equipment is even more obvious in small tanks) we use inline atomisers in most of our setups. In the 9.5 gallon (36 litre) tank above, both the surface skimmer and glass outlet are hidden behind the hardscape, and an inline atomiser is used to inject CO2 into the tank. Without the need to remove any equipment, the tank has a very clean look.

Inline reactor

Reactors aim for a 100% dissolution rate.

They are very efficient compared to atomisers/diffusers and are the ideal choice for people who can't stand CO2 mist in the tank. Their efficiency and dissolution rate will depend on the design of the particular reactor and the flow rate of the filter/pump driving it. They can be used on filter outlets, but they do reduce flow slightly.

Reactors can only handle a certain maximum injection rate for their design/size - so getting the right size for your tank is important. If your tank is larger than 250 litres, most commercial reactors that are shorter than the length of your arm will not be optimised to dissolve CO2 at a high injection rate into your tank. You would need to look for specialist retailers or DIY reactors that are larger in size, coupled with a way to get enough current through them. Many people who buy small commercial reactors for their large tanks end up not being able to achieve good saturation rates.

DIY reactors (Rex Grigg etc) can also be made cheaply using PVC tubing. They are not much more expensive than atomisers, despite the apparent bulk.

The only disadvantage of using reactors is that they do not produce CO2 fog, which plants can use directly. Many tanks grow well without fogging, so this isn't a major concern. Reactors should be the default choice for tanks if you do not want mist.

Advantage of CO2 Mist

The main disadvantage of misting is that it can be unsightly if bubbles obstruct the view of the tank.

The upside is that, according to data from controlled experiments published in the Barr report, plants can use CO2 fog directly for photosynthesis. This allows us to supply CO2 to plants above the maximum saturation rates of dissolved carbon dioxide that can affect animal respiration - this is the main advantage of CO2 fog. The Barr report reported a 25% increase in oxygen production (used as a proxy for photosynthesis) in the sample that had access to CO2 fog - compared to the sample that did not; even though the two test samples had the same level of dissolved CO2 in the water column.

Misting is not a necessity in most tanks, but we find it gives an advantage with difficult species or plants weakened by poor transport etc. For most tanks, achieving a good level of dissolved CO2 is sufficient to grow most species.

Another way to look at CO2 fog is that it allows you to have a lower dissolved CO2 rate while still getting carbon dioxide to the plants. This particular scenario may apply if you have very sensitive animals that prefer a lower level of dissolved CO2 in the tank.

The importance of flow

Especially for mist users, flow patterns in the aquarium greatly impact the efficacy of CO2 delivery. Some clues that you need to adjust / improve flow:

Uneven Growth

Some areas plants grow very well, while some areas have very poor plant growth, despite getting similar amount of light & fertilization. This is more obvious with difficult plants - which usually do visibly better in open areas with good flow, and do less well along the aquarium sides.

Confined Mist

The CO2 mist is confined to a specific area, or you observe that the mist from the diffuser/atomiser rises vertically from the diffuser.

Diffusers should always be placed in the downdraft of the water outflow stream; bubbles should be fine enough to be pulled down by the stream when they first exit the diffuser.

There should be good distribution of the mist, otherwise it may indicate that the diffuser/atomiser isn't producing a fine enough mist, or that the flow is too weak.

Algae in specific areas

In many setups, certain types of algae are found in certain areas of the tank.

For BBA, this may indicate areas where the flow is too turbulent (with CO2 mist). Installing outlets that provide a more even distribution may help.

Cladophora and hair algae can accumulate in areas where detritus accumulates due to the flow pattern. Planting more robust plants in problem areas and doing more cleaning to remove detritus build-up can help. Planting more robust plants in areas of poor current - where demanding plants don't grow as well - is one way of overcoming current problems.

Using Hang-On-Back Filters

The HOB outlet should flow horizontally rather than down to travel the length of the tank. The down wash flow should be strong enough to push the CO2 bubbles against the substrate and allow the mist to travel the length of the tank back to the filter inlet area.

This will not work if (a) the diffuser is not producing fine enough bubbles, (b) the flow is not strong enough, or (c) the flow pattern is wrong (HOB flow is down instead of horizontal).

The same applies if canister outlets are used instead of HOB.