Your Pond Filter needs oxygen. Your Koi or Gold Fish Need Oxygen. You Need A Pump & a Water Fall Or Fountain

How Does Oxygen Get Into Water? Important Pond Keeping Concept here

Let's think of a lake large or small it doesn't matter. The surface of the water in contact with air on a calm day can easily be calculated. All oxygen enters the water through this surface which is in constant contact with the air ... all air contains about 21% oxygen (decreases with altitude) and only a small portion of this can get transferred to the water. This transfer of oxygen is slow and minimal and more area allows more transfer to take place.

On a calm day in a lake the amount of oxygen in the lake would be lowered because fish and other aquatic life including a vast myriad of bacteria types and other micro-organisms consume oxygen just like you and me in order to survive.

Now if the day is calm and the water very warm even less oxygen would be dissolved in the water because warmer water cannot hold as much oxygen as colder water (this is why trout need cool streams and lakes to live in because they need more oxygen than say carp or bass) to allow them to live and breed and  catch prey through rapid movements.

What this means in practice is that long periods of calm days combined with high water temperatures is bad for aquatic life and the rate of death for all living creatures in the lake increases. The same thing happens in your pond.

Now consider what happens on a windy day in a lake ... suddenly the surface area is no longer flat but is covered with small waves and this has the effect of dramatically increasing the surface area in contact with the air and quite simply more oxygen can now move across this barrier and become dissolved into the water.

In a pond it is difficult to get much of a wave effect since the system is small and often sheltered as well. We compensate for this by adding a sprinkling type fountain or better still a waterfall to a pond pump circuit. These are just 2 ways of increasing the surface area of the water in contact with air so that more oxygen can be naturally transferred to the pond water to make up for the oxygen consumed by the inhabitants of the pond and most importantly by the bacteria in the pond filter.

There is another way to achieve the same result and this is seen more often in aquariums than ponds .... an air pump is used to bubble air continuously through the pond water. These air bubbles are the exact equivalent of the calm lake and air interface I described ... the surface area of the bubble is water and the inside of the bubble is air and as the bubble ascends up through the water some of the bubbles collapses freeing the oxygen enriched water into the mass of the pond water.

The smaller the bubble the more oxygen can be transferred this way.

During winter and when a gale is blowing the oxygen levels in a lake would be at its highest. The interface for air transfer into the water is at a maximum - waves rise, crash, break along the bank and continuously present a new air/water interface across which air can be absorbed by the water. And because the water is colder it can hold more oxygen.

You can't create a gale in your pond but you can offer an alternative such as a fountain or water fall.

So what has this got to do with pond keeping?

Oxygen in water is used by plants fish AND bacteria.

A pond filter is absolutely essential in almost all ornamental garden fish ponds to purify the waste products excreted from the fish. In this way the fish are prevented from poisoning themselves to death. A Pond filter can only work if it gets oxygen to feed its own bacteria 24 hours per day 7 days per week. Just like you and me pond filter bacteria die if they do not have air (oxygen) to breathe. This oxygen is contained in the water being pumped through the pond filter.

Stop the pump, then you stop the water flow and after about 5 or 6 hours all the bacteria in the pond filter are DEAD.

The bacteria survive a while because there is normally a bit of water left in the filter. Bacteria use this as their emergency oxygen supply. This is why we say after about 5/6 hours the bacteria die and not immediately the pump is stopped. Once the bacteria die and you continue feeding the fish toxic chemical build up in the pond water occurs.

Do this when you have a highly stocked pond, the water is warm, possibly full of algae and you have a recipe for potential fish mortalities.

The conclusion: