An interesting question has been asked: “If an air pump is turned off for an hour in the evening, does the dissolved oxygen in the pond water disappear immediately the air pump stops?” (The pond has an aerated bottom drain and I assume the air is being turned off so that the koi can be seen more clearly). I could approach the answer from a technical point of view but it is much simpler to draw a comparison with a situation that we all have experienced and can relate to.
A pond can be compared to a room that has an open window. Assuming there is no wind to influence the situation, the slow, but steady, air flow through this window will provide adequate ventilation for a small number of people. As the people in the room breathe in oxygen and breathe out carbon dioxide, the level of oxygen will decrease, and the level of carbon dioxide will increase. With only a few people in the room, this change will not be noticeable and they will breathe comfortably. The larger the area of the window, the greater will be the number of people that can occupy the room.
In comparison with a pond, the open window is equivalent to the surface of the pond. The greater the surface area, the greater will be the number of fish that can live in it using just the natural gas exchange at the surface to provide them with oxygen and to remove the carbon dioxide that they produce. In a lightly stocked pond with a large surface area, there will be enough exchange of gasses through the surface area alone and additional aeration will not be needed.
If you want to increase the number of people that a room can hold, you could blow air into the room by means of a ventilation fan. This will not only introduce more fresh air into the room, but in doing so, it will also push stale air out of the window and prevent carbon dioxide from building up. The more powerful the fan, the more people will be able to use the room in comfort. This would be the equivalent of putting air stones into a pond. The faster the air is introduced through these air stones, the faster the oxygen in the water is replaced and the faster the carbon dioxide is removed, so therefore, the greater the number of fish the pond could hold.
If you were to turn off the ventilation fan when the room is full, the occupants will not immediately feel the effect but, after a while, as the oxygen is being used up, they will find it harder to breathe. There will still be some ventilation through the open window, but this may not be enough to prevent the occupants from suffocating. It all depends on how many people are in the room, as to whether the open window will supply enough fresh air for them to breathe in comfort, or at least survive until the fan is turned back on. Whether the room is being used for aerobics or for yoga classes, and the number of occupants are factors deciding for how long the ventilation can be turned off. Similarly, in a pond, active fish consume oxygen at a greater rate and also, the more fish there are, the shorter will be the time they can be left without additional aeration.
If you turn off an air pump for any length of time and don’t want to spend that time doing regular oxygen tests, then watch your fish. They will tell you whether or not they are finding it hard to breathe. If they are carrying on as normal, everything is fine. If they start showing greater or more rapid gill movements, or if they spend more time at the surface, they need more air. As a “rule of thumb”, with good aeration and normal stocking rates, there will be a reservoir of dissolved oxygen in the water that will be adequate for them to breathe for an hour or so after the air is turned off.
A few points you should bear in mind. The warmer the water, the less oxygen it can hold.
Saturation values of oxygen in fresh water at sea level
(Maximum level of oxygen a pond can hold)
Koi are happy with an oxygen level of 7 to 8 mg/L (7 to 8 ppm). They need a minimum oxygen level of 6 mg/L to be comfortable, and they will die quickly if the level falls to around 3 mg/L. So don’t push your luck! As soon as the first fish looks slightly “out of breath”, the oxygen level will be dropping below the lowest acceptable level, so turn the air back on immediately and don’t leave it off for anywhere near as long next time.
It is a sensible idea, if you feed koi when the air is off, to bear in mind that as the protein in their food is digested, they produce ammonia. The nitrifying bugs (nitrosomonas and nitrobacter) in the biofilter will soon get busy converting this into nitrite and then into nitrate. In order to do this, they will use a huge amount of oxygen, which they will also take from the water. Unless there is sufficient continuous aeration in the filters, the water returning to the pond will be depleted of oxygen. As this mixes with the rest of the pond water, it will dramatically shorten the time during which aeration in the pond can be turned off. It is never a good idea to turn off the air supply to the biological chamber of the filter unnecessarily.