At some time we are likely to have asked questions such as ‘ How do they keep their water so crystal clear?’ or, ‘what are they doing that I am not?’ when we have seen truly clear water.

Some of the most impressive koi collections I have seen have been made even more stunning by the sheer clarity and sparkle to the pond water. I think that probably most of us at some time have been proud of the clarity of our pond water only to visit an even clearer pond, where it may be 6 feet deep and yet it is still possible to read the manufacturer’s name printed on the bottom drain!

What are the secrets of achieving crystal clear water and is such ultimate water clarity within the grasp of any koi keeper?

Let’s consider what causes cloudy water. Most instances include where we can still see the pond bottom, but it is has lost its sharp focus.

That last elusive 10% of pond clarity is often the most difficult to achieve — yet it makes all the difference to the finished appearance of a pond. Assuming turnover rates are correct and solids are being removed efficiently, several common causes of cloudy water should be considered.

1. New and Unbalanced Pond

New ponds frequently struggle with clarity issues, most commonly due to immature filtration and unstable water chemistry.

In newly established systems, overfeeding or overstocking before the biofilter has fully matured often leads to elevated nitrite levels. A nitrite spike is commonly accompanied by a milky or hazy appearance. If visibility suddenly deteriorates, testing for nitrite should be your first step. In most cases, once the nitrite issue is resolved, the water clears naturally.

Another factor in immature systems is insufficient biological film on filter media. As bacteria colonise the media, they produce a sticky substance known as zoogleal film. This slimy layer does more than support biological filtration — it also helps trap microscopic suspended particles, effectively polishing the water. In new filters, this film is not yet developed, so ultra-fine particles remain suspended.

2. Algae

Suspended single-celled algae can quickly turn pond water into “pea soup.” The most reliable solution is installing an Ultra-violet clarifier inline between the pump and filter. UV light causes microscopic algae to clump together so they can be removed mechanically by the filter.

If a milky hue reappears despite using a UV, possible causes include:

a) The UV bulb is nearing the end of its effective life (most require replacement every 12 months).
b) Prolonged hot, sunny weather has accelerated algae reproduction beyond the UV’s capacity. In such cases, clarity may improve naturally with cooler weather, or a higher-capacity (or additional) UV unit may be required.

3. Food-Related Cloudiness

Fish food can also contribute to reduced clarity.

Cloudiness may occur after changing brand or diet. Poorly bound ingredients can break apart once submerged, releasing fine particles into the water. Certain formulations may also affect digestion efficiency, altering waste characteristics.

Additionally, some foods contain colour enhancers or dyes that can leach into the water. This may subtly tint the pond and even affect the perceived brightness of koi whites.

If clarity deteriorates soon after a dietary change, the food is a likely contributor.

Improving Water Clarity

If water turbidity is not caused by nitrite or algae, the solution lies in enhancing mechanical filtration.

The limiting factor in most filtration systems is solids removal — particularly during summer when feeding rates are highest. This is why ponds are often clearest in winter. If the smallest suspended particles are not being removed effectively, true crystal-clear water will remain out of reach.

Ways to Enhance Solids Removal

Solids can be removed through:

• Settlement chambers
• Filter brushes
• Foam or matting media
• Flow modification using vortex chambers or weir/baffle systems

Mechanical filtration can be improved by increasing media capacity or adding supplementary systems.

One of the most effective polishing methods is a sand pressure filter — commonly used in swimming pools. These units excel at trapping extremely fine particles and can dramatically improve clarity. They require a powerful surface-mounted pump and are typically installed as a dedicated polishing system separate from the main biofilter.

Because sand filters operate mechanically rather than biologically, they can be run intermittently as needed, making them a flexible solution for achieving that final, highly desirable level of crystal-clear pond water.