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Cycling a fish pond - Guide and instructions

Building a fish pond is a long and quite often expensive project, spending all of that money and time for your fish pond to crash before it has even started to display its full benefits can be heart breaking, this is why it is so important to cycle your fish pond and give your fish and the actual fish pond the best start it could possibly have.

The term “cycling” refers to the nitrogen cycle which is natures way of converting detritus and waste into beneficial nitrates for plants and other organisms, this cycle takes place everywhere land or water. To understand the importance of cycling your fish pond it is also important to understand how the nitrogen cycle works.

All organic compounds decay and from the decaying process produce toxic substances such as ammonia, in a fish pond the decaying compounds can be leaf litter, dead fish or invertebrates, uneaten fish food and such like, in the fish pond the toxic substances have nowhere to go so a fish pond filter is added to convert these toxic substances into less toxic compounds and eventually nitrates which plants will consume to aid their growth.

To successfully be in control of the cycle occurring in the fish pond you need to know at which stage the cycle is running at and the levels of ammonia initially in the water column, this can only be monitored by using a reliable testing kit, during the cycle testing the water should be performed on a daily basis until the cycle is complete, this usually takes between 4-6 weeks. The fish pond filter will require colonies of beneficial bacteria, these are housed on the filter media so choosing the correct media is very important, the larger the surface area, the larger the colonies that the fish pond filter can house. Ceramic rings and bio-balls are the most popular choice for the filter media, they are designed to have a large surface area and suit the bacteria for colony growth.

Waiting for the colonies to grow in their own time is a long process, starter cultures are available to speed up this process and are added directly to the fish pond, as the water passes through the fish pond filter the bacteria attach themselves to the media and start working on converting the ammonia to nitrites which are less toxic to the fish but can still be deadly if allowed to rise to unacceptable levels. It is recognised that ammonia levels of 1.0 ppm or higher are lethal to live stock in the fish pond, no fish should be added while ammonia is present or nitrites are present, it will result in an early death for the fish in the worst scenarios but there will definitely be stress placed on the fish resulting in secondary infections at least.

The bacteria that convert the ammonia into nitrites are known as “nitrosomas bacteria”, they require ammonia to thrive so a source must be provided if there is no decaying matter in the fish pond. Pure ammonia can be added to the water ( this must not be scented ammonia), this is readily available from most hardware stores or some keepers will simply throw food into the fish pond and allow it to decay.

The ammonia levels will rise before they start to drop, the peak level should be at 5 ppm and once these levels start to drop it means that the cycle is on its way to completion, as the ammonia levels drop the nitrite levels should start to rise.

A different group of bacteria will control the secondary stage of the nitrogen cycle and these are known as nitrobacters. Nitrite levels are not as toxic as ammonia levels but they can still be deadly if not controlled. This group of bacteria convert the nitrites into nitrates, partial water changes and pond plants will remove most of the nitrates in the fish pond. Like the ammonia, nitrite levels will rise to a peak before they drop, levels over 1.5 ppm are unacceptable once the cycle is complete, if these levels occur then there has to be an unbalance in the fish pond.

As the reading for the nitrites peaks and starts to drop again you should start to get readings for nitrates, nitrates are a food source for many plants and are nowhere near as toxic to fish as ammonia or nitrites, it is stated in various reports that fish can survive with readings of nitrates of up to 100 ppm. I certainly would not be happy with readings this high and in an ideal world the readings for the nitrates would be at zero, in reality readings 10-20 ppm are normally the case.

Lowering the nitrates in the water can be performed by adding lots of aquatic plants to the fish pond, they will thrive on the nitrates and grow healthily, the other way to lower the nitrates is to perform partial water changes.

Think of the fish pond as a very large aquarium, most aquarium keepers will perform a water change of 10% weekly or 20% on a fortnightly basis dependant on the nitrates present. They follow the same routine that fish pond keepers use but with a fish pond these partial water changes tend to be 3-4 times through the summer period when their fish are consuming larger amounts of food etc.

Keepers of planted aquariums perform less water changes, some keepers of planted aquariums never change the water as the plants are doing the work for them, al of these principles apply to fish ponds and aquariums but on a different scale. If there are not enough plants in the fish pond and excess nutrients are left in the water column it can cause outbreaks of algae which can soon take over the fish pond, getting the balance right does prove to be important in the long term. Running fish pond oxygenators alongside the fish pond filter will also help to keep the oxygen levels in the water column high enough for the fish, the plants will release some oxygen back into the water but often this is not enough to maintain high enough levels.

One the cycle is complete you should be getting readings of zero for ammonia and nitrites and low readings for the nitrates, this means that the colonies of beneficial bacteria housed in the fish pond filter have grown large enough to cope with the ammonia levels that adding any fish to the fish pond will produce. Between the period of the cycle completing and adding your first fish it is wise to keep feeding the beneficial bacteria with ammonia so that the colonies do not start to starve and diminish.

Testing Kits

The testing kits are usually sold in two main varieties, test strips or liquid test kits. The choice of which to use is purely personal choice but I have used both over the years and can only offer you my personal views.

The test strips will give you a full reading for various parameters with one dip of the strip in the fish pond water. They can cover the ammonia, nitrites, nitrates,general hardness and pH but I do find them to be less accurate than the liquid tests as they do cover a broader range with each test. They can also work out more expensive to purchase dependant on how many tests you need to perform so generally I much prefer to use the liquid test kits.

The liquid test kits contain various bottles of testing fluid and each one covers different parameters that you are testing for i.e. ammonia, nitrites and nitrates. There are some kits that have a wide range of testing fluids but these are more for the saltwater aquarium keepers who have to monitor the trace element levels as well in the aquarium. All that you need is a basic test kit that covers ammonia, nitrites, nitrates, pH and GH.

With the liquid kits come a few test tubes that hold the fish pond water and the testing fluid is added. Each test tube will have a fill mark and the number of drops required from each test vial is marked on each bottle. The water is then checked off against a colour chart and you simply place this card against the test tube and the matching colour will then give you your result.

There are a few points to bear in mind when using the test kits

  • They all have a use by date, this will be a matter of several months but never use any testing fluid that is out of date it will give you false results.
  • Make sure that you are accurate when you are filling the test tube with the pond water, make sure you only fill to the line on the test tube or yet again you will get false results.
  • Always read the manufacturers instructions and follow them accordingly, different tests will require a different number of drops adding to the test tube and in some tests it may involve adding more than one fluid for each test.

Even when the cycle is complete it still pays to periodically check the water for ammonia, nitrite and nitrate levels. These can change unexpectedly and if you are pre warned of rising values it will give you more time to sort out the problem.

Keeping a check on what is happening in your fish pond should guarantee success with your fish keeping, don't forget- clear water is not always a sign that everything is perfect!