Surface agitation & gaseous exchange in CO2 injected tanks

The value of gaseous exchange

Gaseous exchange restores a tank's dissolved gas levels to atmospheric levels in accordance with the physics of gas pressure. For aquarists, this means replenishing depleted oxygen levels in the tank, as well as removing CO2 from the tank's water if it is at elevated levels compared to equilibrium. Gaseous exchange between the tank's water and the atmosphere is determined by:

Oxygen Consumption

Both animal and bacterial activity consume oxygen, and it's often one of the most overlooked components in a planted aquarium - people assume that plants will oxygenate the water. This is true to an extent. Plants do oxygenate water, but only during the light hours when photosynthesis occurs and when they have access to sufficient CO2 (one molecule of CO2 is required for each molecule of O2 produced), whereas oxygen consumption occurs throughout the 24-hour period.

Two aquariums with the same volume of water: the taller tank has less surface area exposed to air, resulting in poor gas exchange. On the right: Circulation of surface water with deeper layers of water is important. Just having a lot of turbulence deep in the tank but no circulation between the top and bottom layers is counterproductive. Having a circular flow pattern, as shown in the diagram, is one of the most effective ways to get better gas exchange. This flow pattern also pushes down on the substrate - bringing CO2 to the carpet plants.

A counter-intuitive approach

Wouldn't increased off-gassing "waste" CO2?

Intuitively, off-gassing CO2 from a CO2-injected tank does not make much sense. But it is an important part of an effective CO2 system. Consider a hypothetical sealed tank with no gas exchange mechanism, meaning that 100% of the injected CO2 goes into the water. Regardless of your injection rate, it would only be a matter of time before the CO2 reached lethal levels for livestock in such a setup - even if you were injecting at a very low rate.

​A higher injection rate means you will reach that level faster; a low injection rate in a tank with no off-gassing of CO2 will also accumulate CO2 in a straight line - reaching lethal levels is just a matter of time.

The key conundrum

How to achieve high, but not excessive CO2 levels?

In a real tank, there is always gas exchange, as well as consumption by aquatic plants. Even in a tank with a poor gas exchange mechanism, CO2 levels will rise in a gradual manner. As CO2 levels rise, the greater the difference between atmospheric levels and the amount of dissolved CO2 in the tank, the more CO2 will be off-gassed. Plants also remove CO2 from the water during light hours.
Thus, when we inject CO2 into a planted aquarium, the CO2 levels rise in a curve that reflects diminishing returns; each additional unit of CO2 injected into the tank contributes less to the CO2 levels in the tank until the rate of injection equals the rate of off-gassing. At this point, the CO2 reaches equilibrium.

All the CO2 that is injected has to go somewhere; it is either contained within the tank or it is off-gassed. When people say that a planted tank should have 35 or 15 ppm CO2, they are referring to this equilibrium level of dissolved CO2 in the water. The graph above illustrates the problem with poor gas exchange; you are faced with the conundrum of either taking a very long time to build up to optimal levels, or exceeding the lethal threshold too easily. It also takes a long time for CO2 levels to build up in tanks - so when you measure CO2 levels is also important.

In tanks with low injection rates, the supply is used up quickly when the lights come on - CO2 levels drop initially due to plant consumption, before building up throughout the day (plants consume more CO2 in the first few hours).

This is counterproductive; there is significant flow, but CO2 levels are not at the optimum point when it is needed most - at the start of the day. Yet this scenario can give rise to a situation where CO2 levels are very high by day's end, with fish gasping at the surface. Hobbyists are puzzled as to how fish can be gasping and yet their plants are not getting enough CO2. 

This is unfortunately an extremely common scenario in the planted tank world when people use low injection rates coupled with poor gaseous exchange.

The perfect Scenario

In a perfect scenario, the CO2 accumulation graph would look like the one below, where the injection rate quickly reaches the optimum level, but then magically stays at that level and no higher.

The question is, how do we get a steeper CO2 accumulation curve that tapers off in a steeper way?

The optimal way to get high dissolve CO2 levels

Good gaseous exchange levels: Having better gaseous exchange allows usage of higher injection rate.

Curve A: As CO2 saturation increases, CO2 is released at an increasing rate, preventing excessive CO2 build-up. This allows for faster CO2 build-up and easier targeting of high CO2 levels without reaching harmful levels.

There is a balance between having good gas exchange - and having too aggressive gas exchange that prevents CO2 levels from building up meaningfully at all.

This can be compared to using a low CO2 injection rate, but allowing the CO2 to build up slowly over many hours. (Curve B) As explained above, there are many drawbacks to this.

The good gas exchange means that the CO2 levels taper off more steeply as CO2 saturation increases. This allows us to use higher injection rates without exceeding the lethal threshold.

4 benefits of higher gaseous exchange

It makes tuning CO2 to a higher level easier

Compared to tanks with low gas exchange, as it gives faster feedback on CO2 levels (one sees the final equilibrium point in a shorter time). Low gas exchange tanks are very sensitive to any increment of CO2 injection and the final equilibrium point is harder to guess.

It allows for a higher margin of error when tuning CO2

This is because as CO2 levels reach higher saturation points, the high gas exchange mechanism prevents them from rising further. Remember the sealed box example - for tanks with poor gas exchange mechanisms, even small injection rates can easily accumulate to lethal levels.

CO2 levels are more stable

A short build-up time means less variability as the optimum level is reached in the same time window each day. Plant consumption is more negligible when CO2 is injected at a higher rate.

Oxygen is maintained at a high level

This is beneficial to the animals and is more favorable at higher CO2 levels. Oxygen and CO2 levels are independent in a tank, ideally we want high O2 and good CO2 in our planted tanks.

3 tools to improve gaseous exchange

For many planted tanks, especially smaller ones that are not too tall/narrow, reasonably good gaseous exchange can simply be achieved if the flow pattern in the tank exchanges the surface layer of water with deeper layers - this usually also provides some surface agitation.

Having a huge amount of flow in the tank environment, but no circulation between the surface layer of water and deeper layers of water in the tank, does not help gaseous exchange much, as gaseous exchange takes place in the layer in contact with the air.

CO2 injection is tricky in large public planted aquariums like this one at Sumida Aquarium (Tokyo, Japan). Note the outflow pipes (top left of the tank) near the surface that circulate surface water with deeper layers in the tank. A less savvy tank designer would have tried to hide the outlets near the back or behind the hardscape (given the Japanese penchant for aesthetics/clean design) - the flow pattern is important enough to take precedence here.

3 Tools that help creating a flow pattern that evenly cycles the top volume of water (especially the surface layer) in a tank with the deeper areas:

Lily pipes and spray bars

The use of Lily Pipe or Spray Bar filter outlets that direct the flow outlet near the surface in a pattern that circulates the top layer of water (which is in contact with air) with deeper layers in the tank greatly improves gas exchange.

The blue X in the diagram below shows the ideal location for the CO2 diffuser; it should be placed on the opposite side of the outlet from where the downdraft will push the bubbles down toward the substrate. You should see the CO2 mist travel across the tank to the side where the filter outlet is located. If the CO2 mist doesn't make it all the way to the other side, it means either the flow is weak or the CO2 diffuser is not producing a fine enough mist.

Surface Skimmer

The use of surface skimmers, which keep the water surface clear of surface film and draw in the oxygen-rich surface layer of water, is also a good method. Another is to circulate the surface layer of water (which is in contact with the air) with deeper water.

We never run tanks without them; they are also easily hidden behind tall stem plants.

Overflow

The other method is to make use of over-flow systems and sumps, which are also provides great gaseous exchange.