Milky water problems in koi pond

I have two separate queries which I would like some advice on please. I have a half in and half out 4,400 gallon koi pond.

The pond is filtered on a gravity fed system into a three chamber filter. Filter dimensions are 8ft x 3ft x 3ft. The first chamber contains 40 brushes with the second and third chambers have Japanese matting.

The Japanese matting chambers are heavily aerated. An Aquamax 15,000 pulls the water from the filter to a 55W UV and then onwards to the two returns, mid-water and a venturi.

Additional equipment is an Olympic skimmer, weed buster and hi blow 40 for an air diffuser in the pond. Filter brushes are cleaned weekly circulation is good, water quality is spot on, and the pond is covered 24/7 with fine netting. The pond is also vacuumed fortnightly.

My problem is suspended particles in the water, which makes the water look milky. This is exaggerated when the air diffuser is on full as matter from the floor of the pond is disturbed. I have previously attempted to take a digital image of the water but it does not show the problem clearly. The best description I can give is that of a milky appearance when you view in from above. It appears a little like when montmorillonite clay has just been used.

I have stopped using clay to see whether this was the cause. Previously the water was crystal clear to the bottom which is approximately five and half feet deep. Now the water is only clear enough to see in about 18inches deep with clarity and I cannot see the bottom at all. I have stopped vacuuming to see whether particles settle but no change is evident. Fine particles are moving through the water and the UV has had a new tube fitted

My second query is that I wish to replace the decking which is on the top face of the block work and was wondering what suggestions you have? The block work is inches thick. I do not wish to empty the pond as the water quality is spot on and the decking hides return of liner.

Dealing firstly with the milky appearance of your pond water, there are several areas that I am drawn to for further investigation – and some factors we can rule out immediately.

Immature unbalanced ponds are a frequent cause of milky water – brought on by the accumulation of ammonia and other pollutants. But as you describe your water quality as being ‘spot on’ then this possible cause can be discounted and you should be looking at a physical (rather than biological) cause to the milky water problem. The two questions you really need to answer are:

What is causing the milky water, and 2. Why is your filter not removing it?

What is causing the milky water?

The probable cause of your water clarity problem is likely to be associated with your use of clay which you state you have used (but have now ‘stopped using’).

Montmorillonite clay is a fine particle clay that when mixed with water in a beaker will form a milky white suspension. Add this suspension to the 4400 gallons in your pond and you will have a less concentrated and yet still milky suspension – depending on the frequency and quantity of clay you are adding to your pond.

The puzzling aspects to this theory though are firstly, the quantity of clay required to make a 4400 gallon pond so milky that you can only see the first 18″ with total clarity and secondly, why your addition of clay has not led to improved water clarity (as experienced by the vast majority of koi keepers who use a clay). It will be interesting to see if your water clarity improves now that you have stopped using the clay.

Before trying to answer the second question, you should also look at whether you have added any other pond treatments immediately before experiencing the clarity problem. Some pond treatments work by interacting with or removing some of the pond’s nutrients (and inevitably interact with other water chemistry parameters). I have heard of a handful of koi keepers experiencing milky pond conditions after using certain crystalline blanketweed treatments that work by binding up phosphates and nitrates. Have you been using any similar pond treatments for the first time this season? If so these may be the culprits.

2. Why is your filter not removing the milkiness?

Assuming the milkiness is clay-related, then you need to investigate why your pond remains milky after treatment unlike the vast majority of koi keepers. Areas for you to look into are:

a. Dead spots in your pond. There may be dead spots in your pond that lead to clay settling in drifts on your pond bottom, only to be resuspended by the curious rooting around behaviour of your koi. You mention that your pond is aerated by a diffuser and your multi-chamber filter is bottom drain-fed. Make sure that your air diffuser is not positioned to encourage a dead spot to form on the pond bottom – something that in fact could be exploited for your pond’s benefit. For example, rather than having a diffuser in isolation on the pond bottom, why not convert you bottom drain to a turbo bottom drain by fitting an airdome conversion to it. In this way, rather than allowing a dead spot to form in close proximity to a solitary diffuser, the mixing action created by an airdome will encourage suspended clay to flow down the bottom drain. Furthermore, you should investigate whether your pond bottom itself has any dead spots, irrespective of your method of aeration.

b. Improve mechanical filtration. Obviously, whatever is causing you milkiness, the particles are so fine that they pass through your mechanical brush chamber. If you could intercept and trap the ultra fine particles that are causing the milky water, you will be well on the way to solving your clarity problem.

A very effective way of fine-filtering water to produce crystal clear water is to use a sand pressure filter (as used very successfully in swimming pools). These are ideal at trapping and removing even the finest of particles without clogging or blocking too frequently. A sand pressure filter requires a substantial surface-mounted pump and can be used successfully as a separate dedicated system separate from your main pump and filter system.

They can be added just as a UV can be added to an existing pond and filter system and because it does not operate by biological means, it can be used intermittently as and when the clarity of your pond water dictates. Alternatively, the more recent entrant to the mechanical filtration market – ‘The Answer’ uses an ultra-fine micron mesh screen that is permanently backwashed ensuring that even the finest of particles do not pass through your first chamber. There has been a great deal of positive feedback concerning how these units (although not inexpensive) have produced clarity that many koi keepers could not have imagined. Furthermore, they are perfect for adding to a multi-chambered gravity-fed system.

c. Add a flocculent. At this early stage in your investigations, the addition of a flocculent to you pond may prove to be your wisest course of action because of the minimal cost involved. A flocculent is to fine particles that won’t settle, what a UVc is to green water that can’t be removed by filtration. By that I mean a flocculent will cause tiny particles to clump together into larger particles that will then either settle out in your filter or become trapped in your filter brushes. The effect will be very rapid (several hours to a day), depending on your turnover) and will confirm, very inexpensively, whether the cause of the milkiness is due to ultra fine particles that your mechanical filter cannot intercept. Why not give it a try?

Regarding your second question, about the removal of decking, it is not easy to advise on something that performs both practical and aesthetic roles in your pond without seeing your pond in situ. However, why not simply replace the decking with your choice of concrete/stone paving (available in many style, finishes and schemes). Paving could butt up to the end of your pond and trap the liner once the decking has been removed. My only word of caution is that there is a risk of cement entering your pond either when the bed of cement is laid for the paving or when the finished paving is pointed. Take care because this could add further to your milky water problems!

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