What causes oxygen depletion? Looking at oxygen like a budget, oxygen depletion occurs when the overall oxygen production is less than what’s consumed in the pond. Too many fish, not enough aquatic plant life or a general lack of aeration can leave your pond oxygen starved.
Aside from the more clear-cut budget perspective there are also other less obvious factors. One of these factors is water temperature. Cold water holds more dissolved oxygen while warmer water holds less
A good baseline to start with is winter time with a frozen pond. Under the ice, the pond water has the largest capacity to hold dissolved oxygen while the fish are at their metabolically slowest time. Slow metabolisms mean less oxygen used. A winter pond is a good time to not worry about oxygen levels in your pond. But then comes spring and summer.
With the warmer seasons come warmer water temperatures which result in faster koi metabolisms and a reduced ability of the water to hold dissolved oxygen. This is when pond owners need to start putting aeration on their radars. So besides warmer temps and higher metabolisms reducing dissolved oxygen, what else can cause oxygen reduction?
1) Decay and decomposition of organic matter like fish waste or leaves
2) Overcrowding of the pond with too many fish
3) Algae blooms/Eutrophication
Let’s explore each of these a little more. When aerobic bacteria (bacteria that use oxygen) assimilate (breakdown and process) organics on the bottom of the pond they too will use up oxygen. If you have a lot of decomposition going on you will see a reduction of dissolved oxygen.
Overcrowding is fairly common as pond owners often try to pack their pond full of fish (or after years of unplanned breeding they end up with a full pond). Lots of fish using up lots of oxygen will result in a reduction of dissolved oxygen as well.
Algae blooms can cause eutrophication. Eutrophication is when a body of water experiences a severe depletion of oxygen due to excessive nutrient loads and dense algal growth. But wait! Doesn’t algae, through the process of photosynthesis, actually add dissolved oxygen to the surrounding water?
Though that IS true, what the standard definitions of eutrophication often leave out is the subsequent die-off of all that algae and the resulting decomposition of this matter at the bottom of the pond. As I mentioned earlier, decomposition involves aerobic bacteria that, during the process of assimilation, use up dissolved oxygen. So you can see the potential for large scale depletion of dissolved oxygen when you have huge algae die-offs in a pond.
And when you couple that with warmer temperatures and overcrowding you get a perfect storm of circumstances for low dissolved oxygen to occur.
When your koi pond doesn’t have enough oxygen, it can induce stress on your fish. This may result in the fish being more vulnerable to things like parasites, disease as well as infections.
Stressed out fish
Its worth briefly reviewing the functioning of fish gills and some important aspects of the interaction between oxygen and water. Okay, I know you didn’t sign up for a physiology lesson on how fish gills work but trust me, this is really cool. The gills are super thin bits of tissue (filaments) arranged on these half circle arcs that lay behind the cheek of a fish.
The blood in the filaments runs forward, toward the front of the fish while oxygenated water runs against it in the opposite direction (due to the fish moving forward through the water). As oxygen-poor blood runs forward against the oxygen-rich water the oxygen from the water jumps over to the gill filaments via diffusion.
Oxygen has a tendency to want to even its abundance out so it goes from an area of high oxygen concentration (water) to low (blood). Its sort of like when a house party gets too crowded and people start spilling out into the yard! Now you might be thinking that you would end up with both blood and water being partially oxygenated when its all said and done but remember that they are both going opposite directions so the water just keeps unloading the oxygen and the gills just keep accepting oxygen like a conveyor belt (as the blood circulates).
As long as the fish keeps moving forward that’s how this system (counter-current exchange) will work and that’s why a fish being pulled though the water backwards can’t get as much oxygen form the water. This counter-current system is so efficient that it can pull dissolved oxygen from the water 3 times faster than we can using our lungs in air AND gills achieve about 80-90% efficiency!
As a reference, a body of water can contain a dissolved oxygen (DO) reading anywhere from 0 mg/l to 18 mg/l. Fish, in general, need at minimum somewhere around 4-5 mg/l for survival. That number will be different for different species and will also change with increasing or decreasing temperatures. Koi will do best at 6mg/l or higher.
To get a better understanding of that change, consider that when your pond temperature goes from 50 degrees F to 68 degrees F, your fish’s metabolic rate will essentially double. That means it needs a lot more food and a lot more dissolved oxygen. Low oxygen levels have several adverse affects including lowered egg production, lethargy and it makes ammonia more toxic to your koi.
In nature, oxygenation of pond water happens primarily in two ways. The first one is via oxygenating plants; the other happens at the air/water interface. The idea is that air from the atmosphere will diffuse into the surface of the pond at the air/water interface. This is based on factors such as air pressure but here’s the issue. This form of aeration is largely restricted to the first few millimeters of surface water.
Pond plants, just like any other plant, will release oxygen into the surrounding water and absorb carbon dioxide. While this is a great natural system there is a limitation. When the sun is up the process of photosynthesis is taking place i.e. carbon dioxide is absorbed while oxygen is given off but when the sun goes down carbon dioxide is given off (as a byproduct of metabolism) while the production of oxygen stops. So the downside is that you can’t rely on plants to oxygenate your pond during the night, so a dedicated aeration system is great to have.
Types of Koi Pond Aeration
Artificial aeration methods can be put into two main categories:
A common and aesthetically pleasing agitator style solution is the pond waterfall. The waterfall will indeed allow oxygen to enter the water and at the same time it will also generate waves, create water movement and thus spread oxygenated water. While the waterfall solution is a good one it may not necessarily be the best one.
It will depend on the particular pond but if your pond is deep enough you may not be getting enough mixing at the deepest depths. Floating pond fountains are another solution similar to the action waterfalls provide. However they too can have the same depth limitations that waterfalls do. They look dynamic and give the pond a spectacular appearance but the ability for oxygenated water to reach every depth of your pond’s water column is limited.
Diffusers are also a common method for oxygenation. Some examples of those systems are the aerator pump and diffuser stone, solar aeration systems, windmill aeration systems, venturi systems etc. This type of system is anything that forces air into the pond and many times that’s done with air forced through a porous material like an air stone.
But how your pond really gets oxygenated may surprise you. When you have air stones at the bottom of the pond and air is forced through those stones the tiny bubbles rise to the surface. So far so good. As those small bubbles rise the teeny tiny surface area of the bubble interacts with the surrounding water and for the short time that it takes for the bubble to reach the surface, there is gas exchange between the water and bubble.
That is one way your pond gets aerated but that’s not the main pathway. As the bubbles rise up through the water column they essentially drag water up with them and when you have a stream of air bubbles all shooting for the surface it creates a constant movement of water towards the surface. This is critical because it brings to the surface the water that is low in dissolved oxygen and when it hits the surface it experiences diffusion of dissolved oxygen from the surrounding atmosphere.
This same water then gets pushed to the side by water coming up from underneath it and then gets dispersed throughout the pond- outward and downward. This flowing cycle is crucial for a healthy and oxygenated pond.
Flow is the goal, stagnation causes problems.
So why does the bubble size matter then if the goal is flow? Why not just have big ol’ bubbles rising up? Its all about surface area to volume ratio. Without getting into Pi equations just realize that smaller bubbles have larger surface-area-to-volume ratios which allows those bubbles to “grab” more water as it rises to the top.
What are the best aeration systems for your koi pond?
There are a multitude of options when it comes to aeration solutions for koi ponds. The idea here is to find a good system that works well and which can provide the proper aeration your pond and fish need. All of the koi pond aeration systems come with their pros and cons, so its important to weigh those in light of your pond situation and your needs. The following is a list of many of those systems and little bit about how they work. The first list will cover the diffusion systems while the second list will cover the agitator style solutions.
Aerator pumps with air stones are an excellent all around option for many koi ponds. They offer excellent upward water flow thanks to the bubbles they generate, and they do end up moving a lot of water towards the surface continuously.
Looking at the process closer, the bubbles exit the airstone and as they rise they grow larger due to reduced water pressure and as they move up while growing larger they create a nice steady flow. Besides water being brought to the surface it also brings things like harmful gases (methane, hydrogen sulfide) from the muck below, carbon dioxide and oxygen depleted water.
It also causes temperature evenness in the pond rather than areas of warm and cold (thermoclines). Like everything else in ponds, there will be some crud buildup over time and the stones will need to be unclogged but you can expect to get a lot of use between cleanings.
I’ve discussed airstones and now lets move on to the pumps that actually get air through airstones and diffusers. There are many different types of aerator pumps out there.
The Linear Piston Air Pump
There are very few moving parts with this design. The piston moves on a layer of air which eliminates the need for lubrication and reduces wear. Fewer moving parts means less to worry about. As a result of this type of design, longer service lives can be expected (up to 8 years). They are also quiet and energy efficient.
Linear Piston Moving Diagram
Summary: • Quiet • Energy Efficient • Oil-less Operation • Long Life
Linear Diaphragm Air Pump
Like others in the air pump industry the linear diaphragm style is also quiet and energy efficient. One can expect 2-4 years of operation before changing out the diaphragm becomes necessary. The change-out process is pretty easy and the parts are not typically very expensive. These too are pumps that don’t require oil to operate. One advantage is that they boast high psi outputs and thats important when running multiple air lines from one unit.
Summary: • Quiet • Energy-Efficient • Oil-less • High PSI Output • Inexpensive Parts
Rotary Vane Air Compressor
Now we get into air pumps that are designed for larger ponds. The rotary vane design is very well thought out. As the vane rotates there is little to no air leakage therefore resulting in a pump that can sustain a given air pressure but run at lower RPMs. There is negligible loss of performance over its lifetime and unlike other types of pumps it will not seize which reduces the need to spend time or money on repair costs.
Summary: • Quite Operation • Long Service Life • Reduced Repair Needs • Maintains Performance over Time
Rotary Vane Pump
These are moving lots of air. With these systems there is only one moving part and its the impeller which is connected to the motor shaft and as it spins it not only touches nothing (no friction with other parts) but it traps air between its blades and forces that air outward and forward into the channels. All of this is to say that this design is super quiet and energy efficient and more quiet than other systems moving the same amount of air. These too are oil-less in their operation.
Summary: • Quiet • Energy-Efficient • High Capacity • Oil-less Operation
Rocking Piston Compressor
Typically associated with larger ponds, the rocking piston compressor is designed for those that need to aerate lots of water. As an example, 1/4 HP unit is designed to aerate about an acre of pond. These units will have permanently lubricated bearings which means it doesn’t need to have oil added later on. The design allows for easy access when repair needs arise though they generally have long service lives.
Summary: • High Capacity • No Additional Oil Needed • Easy Access to Repair • Long Service Life
Rocking Piston Air Pump
Moving on from the actual pumps themselves to various ways of powering those pumps and injecting air into your water, lets get into windmills and solar power.
Windmill Aeration Kits
The great thing about using windmill aeration kits is that these do not require any electricity. Instead, they run only based on the wind power. If your koi pond is remote, you may want to explore this option because it works amazingly well. The air that gets pumped runs though airlines and finally through a diffuser like an airstone. With windmill aeration you don’t have to worry about the energy bill or usage. It can be a solid option for those remote koi ponds.
Solar Powered Aerators
This type has the obvious benefit of drawing its power from sunlight rather than from the electrical grid thereby saving you money. These systems are set up on a battery pack that runs the aerator but is charged via solar panels. In times of limited sunlight they can still run for some time. The actual business end is some form of diffuser (like a diffusion ring or a diffuser stone). They are a nice “set and forget” aeration solution.
While its not a solar panel or a windmill, the venturi is still a method for getting air into your pond. A lot of people use venturi for their koi pond aeration, and it’s easy to see why. Not only do these operate from the existing pumps, but at the same time, they utilize a physics trick that allows you to draw the air in via the action of flowing water.
They do an excellent job at being a backup for the main aeration system, and they can even be your primary system if necessary. However, they are a bit hard to install even if they are inexpensive.
There are many ways to create tiny bubbles by forcing air through a material. Presented here are the main styles that are on the market today.
I’ve touched on airstones a bit previously but let’s dive deeper. This type of diffuser is perhaps the most common one and probably the first one that comes to most people’s minds. Airstones are usually made of glass-bonded silica and, when air is forced through them, create nice, fine bubbles (the best kind!). Lots of small bubbles means that each bubble not only helps to add much needed dissolved oxygen to your pond but each little bubble inadvertently pulls water from the bottom toward the surface.
This little action multiplied by a gazillion little bubbles really adds up to lots of water circulation in your pond which is a good thing. Something that lives in your pond and has lots of little crevices is bound to gum up. Airstones are no different. Expect to do a brief acid bath for your stones about once a year.
These are typically round in shape and contain a flat membrane with thousands of small holes in it that force the air into taking a small form. Sometimes there will be one disc, sometimes multiple discs on the same platform.
This is certainly something you will want to consider. Luckily manufacturers have taken this into consideration and most of them are pretty quiet. The sound they produce is often described in units called decibels or db for short. So when you see an aerator pump described as 60db then you know that it sounds roughly like a busy office.
Here is a chart that gives you an idea of how loud decibels are in comparison with everyday sounds.
Amount of air pumped out. This is often described in CFM or LPM. Essentially, this is a unit of measure for how much air is being pumped out of the unit. CFM means Cubic Feet per Minute while LPM means Liters Per Minute. So how do these compare if you need to do a side by side comparison of two similar units? To put it simply, 1 CFM equals 28.32 LPM. So for example if see an aerator pump advertising that its 3 CFM then that translates to almost 85 LPM.
You will notice that a lot of aerator pumps will specify how deep they can realistically pump air to. Pumps have limitations on how deep air can be effectively pumped to but fortunately those depth maximums are often much deeper than most ponds are. Its important to be aware of that when looking at various pumps to buy for your particular situation.
Cost of Operation
You will notice that these aerator pumps are listed by their wattage. Watts, a unit of power, tells you how much power is being produced out of that air pump based on the amount of electricity coming into it. The watts is directly related to its power output and to the cost. Here is a good resource for understanding the concept further.
The calculation is pretty simple though. Its watts times the # of hours run per day then diving that by 1000. Take that number and multiply by 30 days then multiply that sum by your particular rate per kWh (national average is about 12 cents per kWh). So let’s say an air pump uses 20 watts. What is the monthly cost on that air pump?
20 watts X 24 hrs = 480
480/1000 = 0.48 kWh
0.48 X 30 days = 14.4
14.4 X 0.12 = $1.728/month
♦ The oxygen levels of your pond water are critical and need to be closely watched (especially when the weather turns warm).
♦ Agitation and Diffusion are both good ways of getting dissolved oxygen to your fish but diffusers positioned at the bottom create the best conditions for oxygen circulation
♦ There are many different styles of aerator pumps and picking the right one for your pond means matching up your needs with what the pump is designed for.
♦ Try to understand the specifications of the pump you are interested in so there are no surprises later. Look at noise output, energy use, air output and depth limitation.