Sand Filters and Koi ponds...

Have you been advised that a sand filter is the way to go to provide good mechanical filtration for your Koi pond?

Sand filters are arguably one of the worst filters things that you can inflict on your Koi.

There is a cure that will help you enormously. Check out the new prefilters. And then convert your sand filter into a bubble bead filter.

Sand filters have no place in the modern happy, healthy Koi pond

I would suggest that there are two reasons put forward to advocate the use of sand filters in Koi ponds

1. Ignorance (by far the majority of cases)

2. Willful misrepresentation in order to close a sale (terribly sad but it happens)

Of the two, option 1 is the better.

Let's see why sand filters have no place in any self respecting Koi pond.

Sand filters appear to have emerged on the Koi scene by virtue of the fact that they work pretty well in swimming pools. Most Koi ponds are built by yes, you guessed it, swimming pool constructors!

I am not laying blame squarely at the door of swimming pool constructors. It is a common enough mistake. But the only similarity that a Koi pond has with a swimming pool is that they both contain a fairly large body of water. This is where any similarity between the two ends.

A swimming pool constructor who also builds Koi ponds should however, know better. If your constructor advocates the use of a sand filter I would suggest that you educate him, or seek better quality advice elsewhere.

The logical progression from swimming pool to Koi pond construction is understandable but a square peg will never fit into a round hole. A sand filter is designed and works in swimming pools because a swimming pool is effectively a sterile environment.

But introduce Mother Nature, as you do in a Koi pond, and all sorts of unplanned for events start to take place in the sand filter, many of which are undesirable, especially if you happen to be a Koi.

Filtration options in South Africa were limited...

Up until now South Africans have primarily used sand filters and vortex chambers to accomplish mechanical filtration in their Koi ponds. This is simply because nothing else has been available. Of these two methods, vortex chambers are the better option but suffer from limitations of size, flow rate and imperfect solids separation (some suspended solids are not removed).

Sand filters suffer from a nasty phenomenon known as channeling.


Inside the sand filter, water is forced through a bed of sand that traps solid particles within the bed and allows only clean water to pass to the bio filter stage. This in itself is an efficient process.

That's the problem - as a mechanical filter - sand filters do work!

In a perfect scenario, the water would be evenly distributed through the sand bed, and optimum efficiency would result. Water however, is lazy. In order to pass through the sand bed it will seek out the path of least resistance.

The reality is that this doesn't happen. After a backwash, the newly 'cleaned' sand bed settles and immediately slowly starts to clog up with solid debris from the pond. This closes off some of the 'channels' that water would ordinarily have passed through and 'preferred' paths of least resistance start to emerge where the vast majority of water being filtered will pass through. This phenomenon is what is known as channeling.

You should be able to see quite clearly that channeling is going to present itself in any situation where water flows through a static bed of media. Think of your bio filter - what media do you have in it? Is it static (in other word it doesn't move)? If so, you WILL have channeling in your bio filter as well!

The efficiency of removing the finer solid particles in the water is not hampered by channeling. However, as the channels 'close off', the water flow rate will drop because the resistance through each open channel will increase as the water flow rate increases. Those of us who have swimming pool sand bed filters know that after a good backwash the flow rate of the filter is improved, sometimes dramatically.

This is why there is a variable pressure drop across any sand filter.

Varying (slower) flow rates have implications for the bio filter stage as if the flow rate becomes too slow, the efficiency of the nitrification process may be impaired. But this is the least of the concerns for now.

Dead spots

So what if water 'channels' through my sand filter?

The biggest problem of channeling is that of 'dead spots'. In the areas where the water is NOT flowing in the sand bed, it is effectively stagnant. Stagnant water rich in waste product - both dissolved and solid - water is a recipe for rotting with consequential disease manifestation, malicious bacteria and pathogens.

In a filter system a bad smell is NOT desirable. It's a sign that something is WRONG! Gunk, gunge and smelly sludge have no place in Koi ponds or Koi pond filter systems.

The build up of these unwanted nasties as well as the resultant toxins that stem from other anaerobic processes taking place in these dead spots leach into the water that is flowing through the sand bed. This ends up straight back in the pond, with all the attendant potential for disaster that comes with it.

Back washing doesn't cure the problem. It may dislodge some of the nasties, and even remove some of the pathogens by doing a complete flush for several minutes. But as soon as normal filtration resumes the colonies that are left attached to the individual sand grains begin their evil work once again. These are fast growing bacteria - you can just imagine what's in the filter after a week.

If your Koi are in anything less than perfect condition, a sand filter is the surest way to test your Koi's immune system against a battery of undesirable elements. And this is a continuous relentless test. I would think that there are easier ways to waste money.

Many a Koi has paid the ultimate price, much to the bemusement of the owner. After all, the Koi 'looked' perfectly healthy and the water 'looked' perfectly clean. But with a sand filter, all it takes is time. Sooner or later disease has a far higher probability of breaking out in your pond, simply because it's always present in the pond water at dangerous levels.

My advice is: If anyone advises you on using a sand filter, point them to this article. A Koi pond is NOT a swimming pool where chlorine can be added to kill everything living in the sand filter. If you're already using a sand filter, get rid of it and the sooner the better.

The moral of the story is this: Koi keeping is expensive, but it is far more expensive if you don't do it properly the first time! Consider the cost of replacing a sand filter or an entire Koi collection in an already commissioned Koi pond...