(Aerating koi ponds)
Why do we aerate a pond? The obvious answer would be to provide the fish with oxygen but, to understand what aeration really does, we need to think about what actually happens when we use an air pump to blow bubbles under water.
Air pumps take in air from the atmosphere so the first thing to understand is what is in the air that it takes in. The atmosphere is a mixture of several gasses; roughly 78% nitrogen, 21% oxygen, 0.04% carbon dioxide plus very small amounts of about thirteen other gasses too. When an air pump pushes air through an air stone into water, it can’t selectively choose just the oxygen to dissolve into it, to some extent, all the other gasses will dissolve too.
Only two of those gasses are important to koi keeping; oxygen and carbon dioxide, so I will largely ignore all the others to simplify, as far as is possible, what happens to water when it’s exposed to the atmosphere or has air bubbled through it.
A “push of war”
If water with no dissolved oxygen in it is exposed to the atmosphere, molecules of oxygen in the air above it create a pressure on the surface. If there are no molecules of oxygen already in the water to push back against them, something I have often likened to the opposite of a tug of war takes place. It can be thought of as a “push of war”. One by one, oxygen molecules will be pushed into the water by the pressure. As the number of oxygen molecules in the water increases, they begin pushing back. Eventually the push from the dissolved molecules exactly balances the push from the atmosphere. The push of war reaches a stalemate and the whole process slows to a halt.
If the water also initially had no dissolved carbon dioxide in it, exactly the same would be happening to that gas too. The pressure of the atmospheric CO2 would also be pushing those molecules into the water until the pressure pushing back exactly balanced the pressure trying to push more in.
After being exposed to the atmosphere for several hours, water that initially had no dissolved gasses in it will contain some oxygen and a trace of CO2. Although, for the purposes of this discussion about aeration we are ignoring the effects of all the other atmospheric gasses, it should be remembered that similar processes would be happening to them too and traces of these will also end up dissolved in the water.
Now add a fish
If a fish is put into that water, its gills will take up some of the dissolved oxygen molecules and replace them with some CO2. Two things now happen simultaneously. With some of the dissolved oxygen having been removed from the water, there is less push from it to balance the oxygen pushing from above so more oxygen will pushed in to take its place.
At the same time, the extra CO2 in the water will be pushing back at the atmosphere with a little extra force, resulting in the excess CO2 being pushed out. With only a few fish in a pond that has a large surface area, the process of oxygen being pushed into the water to replace what the fish have used, while at the same time, the waste CO2 pushing its way out, will keep dissolved oxygen and CO2 at normal levels. With a large enough surface area and a limited number of fish in a pond, this natural process will be all that is necessary to sustain the system.
Adding more fish
The push of war of dissolved gasses into and out of water with fish in it is called diffusion and there is a finite rate at which it can happen. In a pond, if this is the only way that oxygen and CO2 can get into and out of the water, the stocking rate has to be low or the fish will deplete the oxygen more quickly than it can be replaced and this will ultimately lead to them suffocating. One way to increase the number of fish a pond could hold would be to increase the surface area in order to provide a greater area where the gas exchange can take place. At first this may seem impossible but this is exactly what is achieved by aerating water with an air pump and air stones.
Increasing the pond’s surface area
Air added through an air stone near to the bottom of a pond breaks into a rising stream of bubbles and if the size of these bubbles is small there can be a huge surface area between the surfaces of all the individual bubbles and the water surrounding them. Much of the air exchange happens as a bubble forms but it also adds oxygen and allows excess CO2 to leave the water into it as it rises towards the surface. A surprising example of just how much a stream of bubbles can increase the effective surface area is the case of a small pond that has a surface area of 5 m2 (square metres). If 1 litre of air is pumped into it through an air stone which turns that 1 litre of air into 1,000 bubbles then the total surface area of all those bubbles adds up to 4.9 m2. Add that additional surface area to the pond’s original 5 m2 and the new surface area becomes 9.9 m2.
In other words, 1 litre of air in the form of 1,000 bubbles effectively almost doubles the surface area of that pond. If the air stone produced a much finer stream of bubbles, say 10,000, then their total surface area is more than doubled to 10.4 m2 making the new effective surface area of the pond equal to 15.4 m2. This much finer stream from just 1 litre of air will have effectively tripled the surface area of the pond. As long as the pump can keep 1 litre of air in the pond constantly rising to the surface, that particular pond could hold three times as many fish as it could without additional aeration.
How much aeration?
Obviously, once the bubbles have reached the surface and burst they are no longer any benefit in aerating the pond so the air pump has to replace them at the bottom at the same rate as they disappear at the top. To put approximate numbers to this effect, consider an air stone placed at a depth that means each bubble has a journey time to the surface of 1 second. In this case, 1 litre of air pumped into the pond will stay in the water for 1 second before it has risen to the surface and left and ceases to have any effect. To keep 1 litre continuously rising to the surface means that it will have to be replaced at the bottom by another 1 litre every second.
This gives an approximate rule of thumb that 60 litres of air per minute pumped into a pond with a surface area of 5 m2 in a very fine stream of bubbles will add oxygen and remove CO2 at such a rate that it will give the pond an effective surface area three times greater than if it had no aeration. This will allow three times as many fish to be comfortable in it as could be kept without the aeration.
Actually, it’s slightly better than that. As each bubble reaches the surface they cause ripples and these effectively increase the surface area a little more. Also bubbles don’t burst immediately on reaching the surface, they float across the water for a few seconds and the thin film of water forming the bubble is exposed to air both from the inside and the outside which further increases the effective surface area of the water in the pond.
Aeration causes currents in water
Aeration can also be used to improve circulation if all the air is introduced in one place, preferably close to a wall as in figure 1. As the bubbles rise towards the surface, there will be a natural upward current of water and as the current reaches the surface it will spread out across the pond. To replace the water being drawn up by the stream of bubbles, more water will be drawn across the bottom towards the air stone. The effect of these two currents will cause a downward current at the wall opposite to the air stones forming a slowly circulating current taking aerated water all across the width of the pond, down the opposite side and back across the bottom ensuring that there are no dead spots in these areas. One air stone on its own won’t create much of a current but several air stones or a ceramic plate air diffuser will create a sufficiently powerful upwelling of water to start this cycle.
An alternative to creating a current to take aerated water across the width of the pond is to use the upward current from an aerated bottom drain to help keep the bottom of the pond clear of detritus as shown in figure 2. In this case, the rising current sets up two rotating currents that help sweep anything on the floor towards the bottom drain so that they can be drawn into the filter system. These twin currents of water also carry aerated water across the whole width as just as in the previous example.
Aeration in biological filters
As the two colonies of nitrifying bugs in biofilters convert ammonia through nitrite and into nitrate, they use a lot of oxygen in the process. To oxidise just a single molecule of ammonia and convert it into a molecule of nitrate, uses just over four molecules of oxygen from the water as well. Bacteria in biofilters use huge amounts of oxygen and, if additional aeration isn’t supplied to the biofilter, the water leaving it on its way back to the pond will be depleted of dissolved oxygen. To avoid this, biological filters should have an air stone either in the chamber itself or just before it so that it is fed with highly aerated water.
The gas exchange that pushes oxygen into the pond to replace oxygen that has been used and pushes out excess CO2 doesn’t just happen at the water surface and at the surfaces of bubbles, it also happens in koi. The highly simplified diagram in figure 3 represents how fish respiration works.
As the heart pumps blood through the gill, the CO2 that it contains is at a greater concentration than the water also flowing across it on the other side of the gill filaments. The push from the CO2 molecules in the blood is greater than that of the water outside and this pushes out excess molecules through the filaments. At the same time the higher concentration of oxygen in the water exerts a higher molecular pressure than the oxygen depleted blood so molecules of oxygen are pushed into the blood. The heart continues to pump the oxygenated blood around the fish’s body so that the cells that need oxygen can use it and replace it with CO2. By the time the blood returns to the gill for a second time, the CO2 level has increased and the oxygen level has fallen so the molecular forces again push out excess CO2 and push in more oxygen for the cycle to begin again.
Of course, this is a highly simplified description of how oxygen is pushed into a fish and excess CO2 is pushed out, the whole process is far more complex involving an interaction between haemoglobin in the blood and an enzyme called carbonic anhydrase. However, the underlying principle of the gas exchange at the gill is exactly the same push of war that happens at the surface of the water where oxygen is pushed in and excess CO2 is pushed out.