The Science Behind Healthy Fish Ponds
Courtesy of Wildlife Trends
It is summer again. It is hot and dry and most ponds are, to one degree or another, going down. Unfortunately, it looks like we are going into yet another drought summer. It’s time to think about options for aerating your pond or otherwise making it less likely that you suffer a fish kill.
First, we must define some terms so we all understand what we, or I, mean when we say ‘aeration’. Now when most pondowners or casual observers say ‘aeration’, they usually mean, I have found, any device or method of moving the water or adding water to a pond. This includes decorative fountains, pumping well water into a pond, running a boat with an outboard motor around the pond to ‘stir it up’, running faucet water into the pond, using an irrigation pump to pump water from the pond and throw it up in the air, etc. etc.
I have found that what most people actually MEAN when they say aeration can be defined thusly: some method of water movement which will prevent a fish kill, due to a lethally low oxygen level, in my pond. At least that is what they want when they call our company and say they want or need or think they need an aerator.
Technically, when well informed persons in the fisheries science or aquaculture world use the word “aeration”, they usually mean “emergency aeration”. It is simply shortened to “aeration” for convenience. However, what fisheries scientists, etc. mean when they say “aeration” I will define here as such, “the transfer of large amounts of oxygen into the water in a short period of time during an oxygen depletion or period of lethally low oxygen such that no or very few fish are killed”. Basically, it means some mechanical contraption will save your fish if you have an oxygen depletion.
In addition to this, much research has been conducted over the years to determine the efficiency rating of various types of aerators so that aquaculturists can operate most efficiently and maximize profits. For example, a given aerator would not be good if it saved your fish but cost so much to operate that you actually lost money on the enterprise. Fortunately, for recreational pond owners, maximizing profit is not a concern.
By the way, this research found, many years ago, and proved that surface agitator type aerators of the ‘paddle-wheel’ design are the best and most efficient aerators. This means they are most effective and have the highest efficiency rating.
I get asked frequently by pond owners, “Don’t I need an aerator in my pond?”. First, I know from experience, when a pond owner asks that question, he/she means “I need an aerator running all the time, 24 hours a day, to put oxygen in the water, otherwise there will be no oxygen in the water and my fish will die.”
To answer that question, we have to revisit fish pond ecosystems 101. First of all, the vast majority of the time – and I mean 99.999% of the minutes in the year – your pond as plenty, more than enough oxygen in it’s water. Especially in a managed, fertilized pond, your pond has far more than enough oxygen to support your fish. The phytoplankton –microscopic algae- produced by the fertilizer, are just like all other green plants: in the presence of light, they undergo photosynthesis – a process which produces oxygen. In addition, water has a fairly good chemical affinity for oxygen, particularly when it is cold or cool. Simply diffusion of oxygen from the air keeps a goodly amount of oxygen in the water.
So, based on this, the short answer to the above question is, “No. Because your pond usually has more than enough oxygen in it. Why would you need an aerator running constantly to put something in the water which is already there, usually in excess abundance?”
However, if a pond owner wishes to install a system to prevent a fish kill from occurring or to save his fish in the event of an oxygen depletion, then we have a ball game. Of course, a pond owner should realize an aeration system is like an insurance policy – you may never actually need it. But, if you need it – you REALLY need it and it will turn out to be well worth the expense. Trying to predict if a pond or which pond will actually experience an oxygen depletion fish kill is sort of like trying to predict where lightning will strike – it is mostly an issue of random chance and mother nature.
There are electric/mechanical systems available which aerate, circulate and/or destratify your pond such that fish kills can be completely eliminated or the chance greatly reduced. Let’s look at a quick list of the most common systems available on the market for aeration, etc. and their effectiveness.
Decorative Fountains: Not true aeration. To be sure, a fountain will transfer some oxygen into the water, if there is an oxygen depletion, but it is not effective or efficient. Should be considered only decorative.
Well Water Pumping: Not aeration. Well water may actually be devoid of oxygen when initially pumped out. At the least, it is very low. Pumping well water into a pond does absolutely no good as far as aeration is concerned.
Pump Sprayer Type Aerator: Consists of a pump which draws water from the pond and sprays it at high velocity out of some opening back into the pond. Complex openings or nozzles are used to improved aeration by breaking up the water stream, thus increasing the surface area which contacts the air. Similar to decorative fountains, but more powerful. It is true aeration but not as effective or efficient as other types. Some pond owners employ a makeshift version of this type aerator with irrigation pumps but usually they aren’t effective because they aren’t large and powerful enough.
Destratification Systems: On the issue of aeration, this is the one which causes the most confusion. Such systems are NOT – I repeat NOT – real aeration. Some of these type systems consist of an on-shore air compressor and hoses with air diffusers at the end, which lie on the bottom of the pond – thus pumping water from the bottom to the surface. Others are floating unit large fans which ‘blow’ or force air down from the surface, pumping water from the surface down to the bottom and back again.
Regardless of the design, destratification systems keep the water in the pond moving, circulating so the pond cannot stratify – separate into distinct layers. This type system is designed to prevent a fish kill due to oxygen depletion caused by a classic ‘turnover’. It is not true ‘emergency aeration’ and will not prevent a fish kill in the event of a lethally low oxygen condition. I know – I have seen it happen. These systems simply do not transfer enough oxygen into the water to save fish during a lethally low oxygen event.
These systems should probably be called ‘water circulation systems’ as that is what they actually do. They pump water from the bottom to the surface, or vice versa, where it outflows across the surface a long distance from the diffuser site. By keeping the water moving and circulating, they prevent the pond from becoming still and stratifying.
On this subject, I know full well that the manufacturers of destratification systems call their products ‘aerators’ and ‘aeration systems’ and they do a disservice to their customers and the general pond owner public by doing so. I suspect they simply gave up trying to explain the difference when the public insists on calling these things aerators. Still, it just ain’t right.
Floating Propeller Vertical Pump Type Aerators: This is a popular, effective and fairly efficient type of true aeration. Several national brands are widely available. The basic design is a floating ring of some material with an electric motor in the ‘donut hole’ and a propeller on the shaft of the motor. The propeller stirs up and vigorously agitates the water, throwing it up into a spray above the pond surface. However, they are most appropriate and applicable for small ponds, in the 3 acre or less size. These are typically small only 1/2 to 1.5 Hp and they must be floated out in the middle of the pond. Therefore, you have the issue of electric cord running out into your pond and some manner of rope or cable anchored to the bottom or tied to the bank. For a regularly fished pond, may not be the most desirable setup. Also, since you generally need 1 Hp of aeration per acre of water, if you have a large pond or lake, it would require many of these to adequately aerate it.
Propeller Aspirator Pump Type Aerators: This is an effective but not as commonly used design of aerator. It consists of a motor with a shaft angled down into the water and a propeller on the shaft, with the motor housing on a float above the water. When the propeller runs, it causes a drop in air pressure, basically draws air down into it and injects air into the turbulence of the ‘propwash’, thereby affecting a high degree of oxygen transfer. The setup looks like or resembles an outboard motor operating at a high angle of tilt. This type of aerator is effective and efficient, but the units are small, in the ½ to 1 Hp range. For a large pond, it would require many to provide thorough aeration and save your entire fish population.
Commercial Sized Paddlewheel Type Aerators: This is the real, true ‘emergency’ aeration aerator and the industry standard. If you were to visit a large catfish farm in west Alabama or Mississippi, for example, this is the type aerators you will see. They are the most effective and highly efficient, plus they are now mostly manufactured as electric, so they don’t require a tractor and diesel fuel to operate. Where millions of dollars worth of fish are at stake, paddlewheel aerators are the standard. ‘Nuff said.
Another thing to consider is when and how long should the aerator run? If you install an aerator, should you turn it on and run it 24 hours a day, 365 days a year? Or 24 hours a day just in the warm season? Or just for the 3 summer months?
This question goes back to the definition of true aeration. Basically, you only need true aeration when your pond has an oxygen depletion or an oxygen level low enough to kill fish. This doesn’t require a complete oxygen depletion (a condition of zero oxygen) since some fish, especially large fish, will die at 1.5 mg/l of oxygen while other fish and small fish will survive that concentration.
So, assuming you aren’t going to be at your pond (or awake) 24 hours a day, 365 days a year to turn on your aerator in case of a fish kill, you need some sort of electrical control unit or system. Fortunately, these days electronic oxygen monitors are available that can measure the oxygen level in your pond’s water and automatically turn on the aerator (s) if the oxygen gets too low. This is exactly like a thermostat system of the air conditioning system in your house. Of course, this assumes you will ALWAYS have electrical power – no chance for power outages.
Recently, some pondowners, and fish farmers, have adopted a different usage strategy with aerators. They are simply setting them to run every morning during the summer/warm months for several hours, regardless of the oxygen level. Some are doing this in conjunction with the oxygen monitor, so the monitor turns the aerators on in the event of an oxygen depletion at some other time of the day.
You may ask why run the aerators only in the early morning. Well, we have to go back to Pond Ecosystems 101….again. If a pond has a plankton bloom, this green algae produces oxygen during the day. However, like all green plants, the phytoplankton switches to respiration in the dark and actually CONSUMES oxygen during the night. In the instances where the plankton bloom is too dense, the plankton plus the fish will actually use up all the oxygen before the next day. Typically, such an oxygen depletion occurs in early morning – just before dawn until a few hours after dawn – until the sun gets up and shines on the water and the plankton can once again produce oxygen.
Still, the oxygen monitors are useful. If, for example, a pond has a paddlewheel aeration system installed but no destratification system, it could experience an oxygen depletion during the day. If the pond is stratified, it could experience a true, textbook ‘turnover’ in the middle of the day or afternoon. Thus, the oxygen monitors would be needed to activate the aerators.
Of course, no system is perfect and completely carefree. The oxygen monitor does require some regular maintenance. The actual oxygen reading probe must be regularly cleaned of hard water deposits and other fouling to keep it reading accurately. The frequency depends on the water quality of the given pond, the density of the plankton bloom, etc.
On the subject of destratification systems, it is worth mentioning that there are definite and proven benefits to destratification and water circulation in fish ponds. First, it has been shown that water circulation can prevent the sudden die-off of dense blue green plankton blooms common in the late spring to early summer. Dr. Claude Boyd (Boyd, Davis, Johnston, 1978) found that blue green algae blooms are susceptible to sudden die-offs from the intense, bright sunlight of summer, as they have no cellular structure to protect them from the sun’s ultraviolet rays. Other water quality factors associated with the onset of summer type weather contribute. This sudden and widespread plankton die off – a ‘plankton crash’ as it is commonly called – can and usually does lead to a lethally low oxygen level, maybe even a complete oxygen depletion, and massive fish kill. It has been found that water circulation prevents this from occurring.
In general, water circulation and destratification improves water quality and increases plankton production and, in turn, fish production. It thoroughly distributes oxygen from the surface to the pond bottom and makes the entire water column and pond environment available for fish usage. Oxygenation of the pond bottom aids in consumption and decomposition of organic matter and prevents the buildup of muck and sludge.
So, don’t get me wrong, I don’t mean to disparage destratification systems, they are greatly beneficial to your pond, your fish and you. We install them and recommend them widely. It is important for the pond owner to understand, however, that a destrat system is not true aeration and will not save your fish in the event of a low oxygen event.
So, if you consider some type of aeration system for your pond, you need to think about these points outlined in this article. Be sure to purchase and install a system of an adequate size and capacity for your pond and one which actually does what you want it to do, whether that be true aeration or circulation or both. You can make your pond ‘fish kill proof’ with the right equipment and system, but it likely ain’t gonna be cheap.
Boyd, Claude. 1990. Water Quality in Ponds for Aquaculture. Auburn University. Alabama Agricultural Experiment Station.
Boyd, Claude, John Davis and Ellen Johnston. 1978. Die-Offs of the Blue-Green Alga, Anabaena variabilis, in Fish Ponds. Hydrobiologia Volume 61, Number 2. Pages 129-133.