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Q+A In Depth: Pond water is clear, but not colourless.

I am relatively new to koi keeping, having installed a new koi pond in March this year. Taking the advice I have read in your magazine, I have been very careful to find a koi dealer who I believe I can trust.

Under their guidance, by choosing a suitable filter that is appropriate to the volume of my pond, and by stocking the pond gradually with koi, I have been successful in maturing my pond without encountering any water quality or koi health problems.

My pH is 8.0 and my ammonia and nitrite levels have rarely ever registered above the lowest possible reading. However, I am still not completely happy with the quality of my water, that is, its appearance. Although the pond is apparently healthy and balanced, it is relatively clear but has developed a 'tinge' and is clear or sparkling as some other ponds I have seen, especially those at my koi dealer's store.

What else can I do to make my water completely clear and colourless, as I remember my pond when it was first installed?

My pond holds approximately 3000 gallons, is bottom drained to a multi-chambered filter containing brushes, flo-cor and then matting. I have twelve 6-8 inch koi that are healthy, happy and growing.

Firstly congratulations on successfully overcoming arguably the most difficult and risky period in any koi keepers experiences, maturing a new pond or (and their first pond and that!) Most of us would be satisfied to have achieved the results you have to date, but you obviously and understandably want to achieve even better results with your water. It can be a little puzzling, as the filter you have installed is obviously functioning well and is supporting your koi in a healthy pond environment. It is carrying out its primary role perfectly well. However, from your description, your water could benefit from additional processing to make it resemble 'the clarity of the tap water' as you describe it in your letter.

It is difficult to make inaccurate recommendation without seeing your water, and I guess even a sample of your water would not help as even a glass of your pond water would look crystal clear. The discolouration would only become apparent when looking through a significant column or depth of pond water.

There are two possible causes for the apparent discolouration problem that you are experiencing.

  1. ultra-fine suspended solid matter that your mechanical filtration is an able to settle, trap or intercept. This may give you the impression that your water is discoloured when in fact it is the impression given by the suspension of ultra fine particles.
  2. an accumulation of soluble contaminants that have started to discolour your pond water.

The solution to your clarity problem will depend on the specific cause.

1. ultra-fine suspended particles.

Standard mechanical filtration is capable of removing the smallest of particles that can be seen by the naked eye. The removal of fine microscopic particles whose presence is only apparent when looking down through several feet of pond water is a challenge for even the better mechanical filter. These particles are so small that they do not have the mass to settle and are so minute that they cannot be trapped by the finest of passive mechanical media. Possible causes include:

a. algae. We know that the microscopic algal cells are too fine to be removed by a filter (otherwise we would be able to remove green water by filtration without the use of a UVc. You do not mention whether you have a UVc installed, but I assume you have. One likely cause of an intermittent water clarity problem is the possibility that your UVc is working at its limit, and on sunny days is not able to cope with the increased multiplication of algae. When the sun subsides, your UVc is able to catch up, clumping the algae that is then removed by your filter. If you are experiencing such an intermittent clarity problem, you may consider installing an additional UVC. Don't worry though because you cannot overdose your pond with UV.

b. general organic matter. If you have ever examined the contents of your mechanical chamber, you would have noticed how the particles of suspended organic matter are so fine that when they settle they don't form a compacted mass but a fluffy sediment that is easily disturbed and re-suspended into the water column. In the same way that the microscopic algae will not settle or be removed by a standard mechanical filter, there will be a proportion of the finer suspended 'mulmy' organic matter that will remain in suspension in the pond. Additional mechanical filtration specifically targeting the ultra-fine particles will be effective at enhancing your waters clarity. The latest and most innovative solution to this problem is 'The Answer' which can be fitted to a gravity fed filter system such as yours. It is a self-cleaning ultra-fine mesh mechanical pre-filter with that will prevent even the finer suspended solids passing through to the bio-chambers.

Alternatively, you could use the water polishing powers of the sand pressure filter (The use of additional mechanical filtration to improve water clarity was addressed in more depth in a Q+A in depth in Sept 2002)

2. Dissolved impurities.

If your lack of clarity is caused by a discoloration to the water itself, then even the addition of the finest screens and most effective mechanical filtration will not lead to an improvement in water clarity. The everyday activities of keeping and feeding koi will unfortunately and unavoidably lead to an accumulation of dissolved substances often referred to as DOC (dissolved organic carbon). Besides being unsightly, an accumulation of dissolved organic carbon in your pond water is just as undesirable as an accumulation of solid organic material in a settlement chamber, as they will both attract bacterial activity. This means that the number of heterotrophic bacteria in the pond will increase bringing with them an increased demand for oxygen, and the potential to load the pond with toxic by-products such as ammonia. So it is in the interests of the Pond's health and appearance that the DOC is kept to a minimum.

DOC enters the pond as a by-product of feeding koi, from the food itself or from the leaching a soluble organic compounds from accumulated debris in the pond and filter. A pond's clarity can deteriorate so gradually that you may not have noticed a deterioration from the ultra-clear and colourless tap water. A good way to check whether your pond water has discoloured without you noticing it is to view any of your white koi and check that they are as white as they should be or is the water adding a tainted filter making them appear off white?

There are two ways of reducing and controlling the inevitable accumulation of the discolouring DOC in a pond.

  1. Regular partial water changes with treated tap water. This uses the principle that 'the solution to pollution it is dilution', and replaces the DOC with purer, cleaner water.
  2. Marine aquarists are very sensitive to any accumulation of DOC in their systems as marine livestock is intolerant of even the lowest levels of dissolved pollution. You will find most marine aquaria fitted with a protein skimmer, a piece of hardware that is designed to strip out the dissolved organic matter from the water. This technology has now been adapted and engineered for the koi pond, performing the same protein skimming action but on a larger scale. A protein skimmer would keep on top of the accumulation of DOC, maintaining the clarity and keeping the water colourless as the day it was first filled with a tap water.
t filled with a tap water.
 

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