Crystal clear pond water

A stunning way of bringing living, moving splashes of colour into a garden is by keeping fish in a garden pond.

The fish that are routinely kept in ponds are brightly coloured, ornamental varieties of their poorly coloured natural ancestors. However, one of the most common problems pond owners encounter is poor water clarity where the turbidity of the water detracts from the overall image of a pond. It also makes it immaterial if the fish are ornamental or not as either way, it can be difficult to spot them

The first impression gained when viewing a murky pond is usually one of neglect and ill health, suggesting that the pond owner may be guilty of second-rate husbandry. However, ponds can turn green and murky very rapidly and algae infested water actually promotes excellent colour and health in fish. Most pond fish are farmed in ponds that are as turbid as the proverbial ‘pea-soup’ to promote healthy and colourful fish.

Green water is caused by algae and is an issue of presentation rather than fish health. If you are in the minority of pond owners who prefer a green pond then you can continue to do so in the comfort of knowing that your fish are likely to be benefiting from their experience.

Causes of poor water clarity

1. Algae – Green water

Algae is the most common cause of poor water clarity in a pond. Millions of single-celled algal cells proliferate and multiply rapidly from spring to summer in response to favourable conditions, transforming crystal clear water into a sea of green.

Two ingredients that are essential for algae to ‘bloom’ are nutrients and light with the whole process being speeded up by temperature.

Any water feature, including a garden pond should be managed just like any other part of the garden where there is a constant battle between good and evil, namely desirable plants and weeds. As any experienced gardener will explain, a sparsely planted, fertile border will encourage the rapid colonisation of unwanted weeds that seem to appear out of the blue and spread rapidly.

They can be prevented by planting desirable plants that will compete for the same nutrients and light as the weeds. The more plants, the less space for weeds and therefore less weeding.

The same principles are true for a garden pond. If left sparsely planted then the abundant nutrients in the water (that enter via tapwater or indirectly via food) and light will encourage algae to bloom.

This is exaggerated in warmer weather and is why smaller, shallower ponds that heat up quickly are those that also can turn green very quickly. It is far easier to keep larger ponds algae-free. In winter, when temperature and sunlight are at their lowest then algae growth is also reduced. It is not surprising to find ponds at their clearest in winter.

2. Soil and silt

Pond water can be muddied by fish stirring up silt and soil from within a pond, preventing it from settling on the bottom. Mature well-planted ponds that may well not have green water problems may have a sediment or silt problem that has accumulated over the years.

As fish waste is broken down in a ‘natural’ pond and plants die back each winter, then the resistant organic matter settles out and accumulates on the pond bottom together with sand and dust blown in over the years.

As the silt layer deepens, it is re-suspended each time it is disturbed by fish (a little like the ‘snow’ in a snow storm paper weight). In extreme cases, at feeding time pond fish may appear to emerge from a dense silty suspension. Silt is a problem in more mature ponds that have not been cleaned out for several years.

Similar conditions can also be experienced in newly planted ponds where the culprits are inquisitive fish, rooting through the gravel on planted baskets. Koi are notoriously inquisitive fish, probing their snouts through the gravel layer into the aquatic soil in search of tasty morsels. A cloudy suspension of soil in the pond water can result.

3. Insufficient or blocked filtration

If a pond is not of the natural type but is recirculated with a pump and filtered to maintain the water quality, then deterioration in visibility may well be due to the overburdening of the filter. Once a filter is blocked or clogged, any debris pumped from the pond returns back to the pond to be stirred up again by the fish.

A filter performs two functions. Firstly the removal of solid material from the water to keep the water clear and secondly to house beneficial bacteria that will breakdown soluble toxic waste material produced by fish.

Care must be taken when cleaning and unblocking the mechanical parts of the filter that the biological and living part of the filter is not disturbed. If the two parts are performed by one area of a filter, such as in the 2 or 3 layers of biofoam in a box filter then take care to rinse the foams in pond water. Tapwater is best avoided when cleaning a filter as it contains chlorine that will kill beneficial bacteria in the filter, leading to a decline in water quality.

Solutions to a clear solution.

In a water body such as a swimming pool, producing crystal clear water is easily achieved as chemical treatment soon kills off all life including algae and excessive filtration traps and removes all solid material.

In such a situation, no regard need to be given to fish or plants which would not tolerate such drastic clarifying measures. Yet with the rapid increase in interest in water features such as pebble fountains and fountain statues, where no fish or plants need to be cared for, then the water can be treated chemically just as in a swimming pool to guarantee algae-free clear water.

The solution to the cause of cloudy water in a garden pond will differ depending on the cause.

1. Suspended silt or soil.

If this is a problem in a natural pond then a possible way of removing the layer of silt is by pumping it out using a pump that can handle solids. If the problem is more extreme then a complete strip down of the pond may be necessary.

Excessive solid waste in a filtered pond may routinely cause a pump pre-filter (depending on the type) to clog, reducing the turnover of the pump. Although called a filter, such a device is fitted to protect the pump and will not filter a pond effectively. Any waste that collects in a pond is best removed from the pond via the pump into an external filter. Some pumps can be fitted with self-cleaning pre-filters but if this is not possible it is wise to stand the pump off the pond bottom to reduce the likelihood of it sucking in excessive settled solid matter.

If the pump can handle finer solids then it should be placed on the pond bottom where it will draw all of the settled silt into an external filter which can be cleaned at will.

If suspended solids are still a problem with an external filter then it is an indication that it needs cleaning. If the filter needs cleaning regularly then a larger filter should be considered.

2. Green water

Just as weeds growing over a border can be treated in a number of ways, so can green water.

1. The balanced or natural pond.

Using the principles of outcompeting algae for light and nutrients, a pond can be planted with desirable and functional ornamental plants.

Green water can be reduced by using a combination of marginal plants that may shade the surface and utilise nutrients and submerged plants that will outcompete algae within the pond. Such plants include ‘oxygenators’ such as Elodea and Hornwort and even lilies whose pads will also shade the water from excessive sunlight.

When balanced and matured, ‘natural’ ponds have very little maintenance (besides pruning) as the plants carry out the water purification, taking the place of a pump and filter. However, such an approach can take years to become balanced and the pond may still go through a green stage before the desirable plants start to win the battle.

2. UV

A much more rapid and even guaranteed way of clearing green water and keeping a pond permanently clear is by using an ultraviolet clarifier (UVC).

A UVC must be pump-fed and installed with an external filter as it creates significant quantities of dead algae that needs removing from the pond water. UVCs come in a range of sizes to suit a range of pond sizes. The pond water cannot be overdosed with UV and it is harmless to fish and plants. They work by burning the algae and making the microscopic cells stick together into filterable clumps. They are so effective that most UVC manufacturers issue their units with clear water guarantees.

Ideally UV bulbs need changing each season and this is best done in the spring as it will do its most effective work from March to September. In fact there is no reason why a UV should be used over winter.

Unfortunately, once the UVC has cleared the water, then other forms of algae may proliferate using the sunlight that can now penetrate through into the pond water. It is quite common for blanketweed to move in after the pond has been cleared of green water, especially if a UVC has been used to clear it. In this situation, the nutrients still abound in the water and another weed will take advantage of the nutrients and the crystal clear water. Fortunately, the use of an effective blanketweed killer will soon control this problem.

2. Chemicals

A range of chemicals are available to control green water problems in ponds. They can be divided into 3 groups by the way they work.

a. Algicides. An algicide is a chemical that kills algae. When purchasing a chemical to treat a green water problem, take care to read whether you are buying an algae killer or just an algae ‘controller’. The manufacture and sale of these effective algicides is highly regulated and as a result only a handful of these effective pesticides are available from aquatic retailers.

The use of an algicide will lead to an accumulation of dead algae which must be removed using a pump and filter. The pond water must also be aerated vigorously to ensure that any decomposing algae does not reduce the dissolved oxygen so that it stresses the fish.

b. Algae inhibitors.

The majority of treatments sold against green water are algae inhibitors that will control or reduce algal growth rather than kill it. They are less effective than algicides.

These compounds work by either retarding growth through chemical interactions or through the use of a dye added to the water that filters out the sunlight that the algae require. With either method, algae growth is reduced rather than killed.

c. Nutrient removers

These are compounds that may be added to the pond water to bind up phosphates and nitrates, taking them out of solution so that algae cannot benefit from them. If used in conjunction with algicides, they can afford long-term effective control of unsightly algae.

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