Pond aeration


Oxygen is the essence of life and is required in abundance in a well stocked koi pond. Oxygen quite readily dissolves passively diffusing from the atmosphere, but this limited amount is insufficient to satisfy the demands of koi and other oxygen-loving pond organisms such as beneficial bacteria and invertebrates in the filter.

Oxygen can often prove to be a limiting factor for fish and biofilters with the health and performance of both benefiting from additional aeration, especially during the warmer summer months.

Dual Purpose Aeration

The most obvious benefit of adding aeration is to increase the dissolved oxygen concentration to its maximum.  This can be achieved in a number of ways using water movement or the injection of air.  Besides ensuring an adequate exchange of gases, aeration is also useful at helping to mix and circulate water within the pond, reducing the presence of dead-spots.

The options:-

1.  Air pump and diffuser
Air pumps are specifically designed for aerating ponds. Working in a similar fashion to an aquarium pump, a pond air pump is engineered to be able to pump larger volumes of air to greater depths.  This is desirable, as to be effective, air must be delivered to the very bottom of a pond to provide the most efficient aeration and mixing.  The koi industry standard is the Hi-blow type pump, which can deliver copious amounts of air down to depths of 8 feet!  Smaller pond air pumps are available which amount to ‘beefed-up’ aquarium pumps.  These are a cheaper alternative to the Hi-Blow, but will not serve as many air stones or no where near the depths of those reached by the Hi-blows. 

These diaphragm air pumps are silent running, and will work tirelessly for many years between diaphragm replacements.  The replacement kits for Hi-blows are considerably dearer than the single diaphragms required for the smaller pond pumps. 

A relatively new appearance on the air pump market has seen the first fully-integrated battery back-up air pump.  This innovative unit constantly senses the power supply, and should it be interrupted, will automatically switch over to its auxiliary battery back-up, which can enable it to pump independently for 8 hours.

2.  Diffusers

The air delivered by an air pump must be used efficiently by an air diffuser, as this will significantly affect its oxygenating performance.  The standard diffuser used by most koi keepers is the golf-ball airstone.  Extremely versatile, inexpensive and durable, these can be placed in the pond or into biological chambers to aid the filtration process.  More recent innovations include porous pipe which can be installed on the pond bottom or the ground breaking air dome. 

An air dome consists of a circular perforated rubber membrane mounted on a rigid backing plate which is connected to an air supply.  The membrane forms a dome as pressure builds, releasing tiny bubbles from each pore.  This ‘self-cleaning’ diffuser really comes into its own when fitted onto a bottom drain as its centralised mixing action improves the performance of the bottom drain.  Ingeniously the air line can even be traced down the bottom drain pipe to leave no trailing air line in the pond to be snagged when netting.


Hi-blow pump – from £150
Smaller pond diaphragm pumps- £20-£30
Air dome £50

3. Water Movement

Water movement can be used to aerate pond water by injecting air into the water as it returns. 

a. Venturi.
A venturi is an adapted piece of pipework which will draw air into a stream of water as it is pumped into a pond from a filter chamber.  Venturis can look very impressive when viewed from the pond side, but when observed underwater, it is evident that the air bubbles are only mixing and aerating the surface levels of the pond water.  They can also have the tendency to obscure the viewing of koi and make quite a noise to boot!

b. Waterfalls
Waterfalls are natures way of improving dissolved oxygen and they can be used effectively to enhance D.O. in a koi pond.  They serve both an aesthetic and a functional purpose but like a venturi will only aerate and mix the upper layers.

In the winter, when D.O. is naturally at its highest and the ponds oxygen requirements are at their lowest, pond aeration, particularly with a bottom diffuser should be ceased.  This will allow koi to settle in the deeper warmer layers of water that will remain stable at the pond bottom.

Kill blanketweed and string algae.