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How to Make Sure Your Fish Pond Water is Always Clean and Clear

Let me start by clearing up a very important misconception about clean ponds. To you and me a pond that is sparklingly clear is clean. To a fish the fact that the water is crystal clear means little and it certainly does not mean its watery living room is clean and healthy. In fact to goldfish and koi a somewhat coloured, cloudy or opaque pond would suit them better so long as it was clean and healthy.

So what do we mean by a clean pond ... viewed from the fish's perspective?

To A Goldfish or Koi A Clean Fish Pond Is ...

A pond that is chemically clean and this means to you and me that all ammonia naturally and continuously produced by all fish is removed by bacterial purification. Quite simply if this removal of ammonia did not take place any fish in a garden pond would have an extremely limited lifespan and by that I means weeks. Do you remember that goldfish in the bowl that you loved so much as a child? Do you remember it lasted about a week before dying? It died because it poisoned itself to death in a clear goldfish bowl that was in fact a toxic trap.

The process of cleaning water to remove ammonia is referred to as biofiltration and this is something a misnomer and is responsible for the basic misunderstanding around pond filters, what they are and what they do.

What is biofiltration and how does it happen in a water garden?

One of the most fascinating parts of pond keeping for the enthusiast is the management of the pond water quality. Highest quality water is probably the most sought after objective. If the water quality is good everything else falls into place. This is the purpose of biofiltration. Note we talk of BIOFILTRATION and not just filtration. There are significant differences and a basic understanding of the difference is important.

Clear water alone is no indication of the quality of the water.

In a swimming pool water is normally pumped through a sand filter using a strong pump that can develop enough pressure to push the water through the fine bed of sand. In passing through this fine bed of sand any solids are removed by trapping. Thus such a filter does the job of removing solids and particles that remain suspended in the water. If there is an automatic pool cleaner the sand filter also removes those solids that settle on the bottom. This is classical filtration namely the removal of solids from liquids.

However the filter in a swimming pool does not purify or clean the water. This job is done by the use of chlorine-based chemicals or chlorine generating devices such as salt electrolysers. In this way the water remains perfectly safe to swim in because the chlorine kills all the harmful bacteria and sanitizes the water. Lots and lots of potentially harmful bacteria would otherwise build up in a swimming pool and many of these would come from inside the filter itself now you know why public swimming baths smell so highly of chlorine type chemicals.

Just think a moment about your sand filter if you have one.

  • If you could see inside and watch exactly how the water flows through the bed of sand you would notice that many areas never see fresh water because they are partially blocked by accumulated debris that had never been able to be cleared by backwashing.

  • You would see that the water flow is through channels rather than across the whole surface of the sand bed. As such stagnant areas develop and in these areas bacteria of the undesirable kind also develop (mainly anaerobic bacteria . ie those that do not need oxygen) survive and multiply. Since chlorine kills all bacteria that find their way into the circulating water this situation is not serious in a swimming pool.

Consider what happens in a pond where no chlorine can be used of course.

  1. Food is thrown into the water, some of which is eaten and the rest remains behind.

  2. That which is eaten by the fish is partly digested and partly expressed as a waste product, which sinks to the bottom of the pond.

  3. The fish needs to urinate and this passes into the water also.

  4. Fish also express impurities (nitrogen based chemicals) from their system out of their gills.

  5. In addition other animals enter into and out of the water also with a need to excrete waste products and on windy days anything can find its way into a pond does so.

Here is an ideal opportunity for pathogenic bacteria to develop and toxic chemicals to build up.

This is exactly what happens in a fish pond, all day every day!

If there were no means of purifying the water the fish would quickly poison themselves.

In nature all sorts of factors combine to keep natural waterways healthy. In a pond however things are different.


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