Gaseous exchange restores a tank's dissolved gas levels to match those of atmospheric levels in line with gas pressure physics. For aquarist purposes, this means matching replenishing depleted oxygen levels in the tank as well as off gassing CO2 from the tank water if it is at an elevated level compared to equilibrium. Gaseous exchange between the tank's water and the atmosphere is determined by:
|Surface Film||Water Circulation|
|The amount of surface area of the planted tank in contact with surface air. Shallower tanks have a much easier time attaining good gaseous exchange.||Whether there is a surface film / oil slick on the water surface||The turnover between the surface layers and deeper layers in the tank. Purely having high flow in deeper layers but with limited exchange with surface layers of water does not give good gaseous exchange.|
Both livestock and bacteria activity consume oxygen, and it's often one of the most over-looked components in a planted tank - because people assume that plants will oxygenate the water. This is true to a certain extent. Plants oxygenate water, but only during light hours, when photosynthesis occurs and when they have access to enough CO2 (for every molecule of O2 produced, a CO2 molecule is necessary), whereas oxygen consumption takes place across all 24hrs.
Two tanks with equal volume of water: the tall tank with smaller surface area exposed to air makes for poor gaseous exchange. On the right: Circulation of surface water with deeper layers of water is important. Purely having a lot of turbulence deep in the tank, but no circulation between top layers and bottom layers is counter productive. Having a circular flow pattern as shown in the diagram is one of the most effective ways to get better gaseous exchange. This flow pattern also presses down against the substrate - bringing CO2 to carpet plants.
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 having an effective CO2 system. Imagine a hypothetical sealed tank with no gaseous exchange mechanism, meaning that 100% of CO2 injected goes into the water. Regardless of your injection rate, it would be a matter of time before CO2 reaches lethal levels for livestock in such a setup - even if you were injecting at a very low rate.
A higher injection rate means that you hit 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.
HOW TO REACH OPTIMAL, BUT NOT LETHAL LEVELS
In a real tank, there is always gaseous exchange, as well as consumption by aquatic plants. Even in a tank with poor off-gassing mechanism, CO2 levels rise in a diminishing manner. As CO2 levels rise, the greater the difference between atmospheric levels and the amount of dissolved CO2 in the tank, and the more CO2 will be off-gassed. Plants also strip CO2 from the water during light hours.
Thus, as we inject CO2 into a planted aquarium, the CO2 levels rise in a curve that reflects diminishing returns; every additional unit of CO2 injected into the tank contributes less to CO2 levels in the tank until the rate of injection equals the rate of off gassing. This is where CO2 reaches equilibrium levels.
All injected CO2 must go somewhere; it is either contained within the tank or off-gassed. When people say that a planted tank should have 35 or 15ppm of CO2, they are referring to this equilibrium level of dissolved CO2 in the water. The chart above illustrates the problem with poor gaseous exchange; one faces the conundrum of either taking very long to build up to reach optimal levels, or over shooting lethal threshold too easily. It also takes quite some time for CO2 levels to build up in tanks - so when you take measurement of CO2 levels is also important.
In tanks with low injection rates, supply is quickly used up as lights turn on - CO2 levels dip initially due to plant consumption before building up across the day (plants consume more CO2 in the first few hours).
This is counter-productive; there is significant flux yet CO2 levels are not at optimum point when it should be most needed - 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 left puzzled how fish can be gasping, and yet their plants are not getting adequate CO2.
This is unfortunately an extremely common scenario in the planted tank world when people use low injection rates coupled with poor gaseous exchange.
In a perfect scenario, the CO2 accumulation chart would be like the one below, where the injection rate reaches the optimum level fast, but then magically holds it at that level and not more.
The question is how do we get a steeper CO2 accumulation curves that tapers off in a steeper manner?
THROUGH HEIGHTENED GASEOUS EXCHANGE
Having better gaseous exchange allows usage of higher injection rate.
Curve A: As CO2 saturation increases, CO2 off-gasses at an increasing rate, preventing excessive CO2 build up. This allows faster CO2 build-up, and easier targeting of high CO2 levels without hitting harmful levels.
There is a balance being having good gaseous exchange - and having too aggressive gaseous exchange that prevents CO2 levels from building up meaningfully at all.
This is compared to using a low CO2 injection rate, but allowing CO2 to build up slowly across many hours. (Curve B) As explained earlier above, there are many downsides to this.
Having good gaseous exchange means that CO2 levels taper off more steeply as CO2 saturation increases. This allows us to use higher injection rates without exceeding the lethal threshold.
It makes tuning CO2 to a higher level easier
When compared to tanks with poor gaseous exchange as it gives quicker feed back on CO2 levels (one sees the final equilibrium point in a shorter amount of time). Low off gassing tanks are very sensitive to each increment in 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 gaseous exchange mechanism keeps it from rising further. Remember the sealed box example - for tanks with poor off gassing mechanism, even small injection rates can accumulate to lethal levels easily.
CO2 levels are more stable
Short build-up time means less fluctuation as it hits the optimum level in the same time window each day. Consumption by plants have a more negligible effect when CO2 is injected at a higher rate.
Oxygen is maintained at a high level
This is advantageous to livestock and is more favourable at higher CO2 levels. Oxygen and CO2 levels are independent in a tank, ideally we would want high O2 and good CO2 in our planted tanks.
For many planted tanks, especially smaller ones that are not too tall/narrow, having reasonably good gaseous exchange can simply be done if the flow pattern in the tank exchanged surface layer of water with deeper layers - this usually also provides some surface agitation.
Having a huge amount of flow within 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 happens in the layer in contact with air.
CO2 injection is tricky in large public planted aquariums such as this one from Sumida Aquarium (Tokyo, Japan). Note the outflow pipes (top left of tank) near the surface that circulates surface water with deeper layers in the tank. A less savy tank designer would have attempted to hide the outflows near the rear or behind the hardscape (given the Japanese's penchant for aesthetics/clean design) - flow pattern is important enough that it takes 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 pipes or spray bars filter outputs that channel the flow output near the surface in a pattern that circulates the top layer of water (that contacts air) with deeper layers in the tank improves gaseous exchange significantly.
The blue X in the diagram below shows the ideal placement for the CO2 diffuser; it should be placed on the opposite side of the outflow, where the downwash of the current presses the bubbles down towards the substrate. One should see the CO2 mist travel across the tank to the side where the filter output is. If the CO2 mist doesn't make it all the way to the other side, it means that either the flow is weak, or that the CO2 diffuser is not producing a fine enough mist.
The use of surface skimmers, which keep the water surface crystal clear of surface film and draw in the oxygen rich surface layer of water is also a good method. One again, circulating the surface layer of water (that contacts air) with deeper water.
I never run a tank without them; they are also easily hidden behind tall stem plants.
The other method is to make use of over-flow systems and sumps, which are also provides great gaseous exchange.