Whether or not you should be concerned depends on the cause of the murk.

Fill a clear glass with pond water. Is it green? If so, your problem is suspended algae. If it's brown, the most likely cause is tannins from leaves or barley straw.

So far, it's not looking good for your crumbling cat litter, Ian.

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After wading through a great deal of heated and not very cogent discussion, I decided that the system probably works as advertised. Were it not for the few who tried it on their own ponds and posted their results, I might have given up trying to sort fact from hyperbole from wild misunderstanding. But everyone who actually tried the anoxic filter and posted about their experience reported extremely good results. Now, I didn't find very many of these people, but there were no experience-based posts at all to suport the aneaerobic-bacteria-will-kill-your-fish crowd. Also, after reading the CD book, I realized that none of the people objecting to the anoxic system actually understood it. Nearly all of the objections are based on incorrect assumptions, for example, that there is stagnant water in the filter, or that the plants do the bulk of the filtration. In the absence of a convincing theoretical problem, and with reports from actual ponds being 100% positive, I'm willing to take a chance. And as you say, we have nothing to loose by trying a new planting medium in our existing ponds . . . unless you believe that evil bacteria live in the hearts of pots of cat litter and that these foul hordes will suddenly charge out to attack any fish unfortunate enough to share the same water.

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As I said, I'm just learning about this myself. But I'll give you a brief (oversimplified) description.

Though the first thing you'll notice about this filter is the plants, it's primarily a bacterial filter. Ponders short on land have put the pots indoors in a tank without plants or sunlight, and it still works. Nitrifying bacteria live in the outer layers of cat litter and convert ammonia to nitrate to nitrite, just like in the more familiar aerobic filters. But in the center of the pot, where there is only a little oxygen, denitrifying bacteria convert nitrate to nitrogen gas. Bacteria in this anoxic zone will also metabolize phosphate which will help control algae. Transport into the pots is accomplished by molecular diffusion and the attraction of ions to opposite electrical charges rather than by water being forced though the substrate. Hence it won't clog (and perhaps the fine clay granules are acceptable).

For a more complete explanation, read this:
http://www.mankysanke.co.uk/html/anoxic_filtration_system.html

I haven't tried this myself, Iain, hence I can't answer your question from experience. As I understand it, the important thing about cat litter is that it retains its crystalline structure. If the material you have turns mushy, it's not going to work. But if the smaller bits are still hard, maybe you've got the right stuff after all. I took the liberty of reposting your question to another forum where the inventor of the system and several who have tried it are participating.
http://www.koiphen.com/forums/showthr ... 2640&posted=1#post1632640

I'd say fact, but poorly understood. Pheromones and pollutants will tend to slow growth when large fish are crowded into a small pond. In extreme cases, the they will take on the short, fat body shape of badly stunted fish. But neither condition is healthy, and when fish "grow to the size of the pond," your pond's ecosystem will be right at the edge of collapse. Hence, when there is any additional stress, for instance, from unusually hot weather or because circumstances kept you from cleaning the filter for three weeks, fish will start to die. It is far better for both fish and fishkeeper if pond is large enough for all the fish at their full adult size.

Snails will eat some algae, but to control it, you'll have better luck with plants. The faster the plants grow, the faster they will extract nutrients from the water. I've read that phosphate is usually the limiting nutrient for algae in fish ponds. If you plant your plants in mesh baskets with kitty litter and a little laterite, the bacteria in the center will also compete with algae for phosphate (and ammonia and nitrate). In the link below, search for "pots" and you'll find planting instructions.
http://www.mankysanke.co.uk/html/anoxic_filtration_system.html

Another way of controlling algae is to use barley straw.
http://www.fishpondinfo.com/plants/barley.htm

For suspended algae (green water), try filtering through several layers quilt batting. Hosing out the batting can be a bit labor intensive, but keep at it and your water will become clear again.

Hair algae is easily removed by winding it around a long-handled brush or gently pulling it to a bucket hand over hand. I prefer the hand over hand method when I can reach the algae to get started. Put a few rocks in the bottom of your bucket so that only a few inches rise above the pond. You'll be amazed at how fast you can clear the pond once you get the hang of it.

I used that product last fall and didn't notice anything. I did make a point of clearing out the dead algae, though.

What has the weather been like there? I just read on another forum that someone in Scotland lost a pond full of orfe, probably because the unusually warm weather led oxygen depletion.

Hi Craig. No claims of being an expert from me either, but I suggest that you stop building now and research the details of your pond. Though apparently simple, a fish pond is a very complex system, and if you don't think it all through ahead of time, you're likely to have trouble later. Many things that are easily done as the pond is built become nearly impossible later, so start with a solid plan and go from there.

If starting from scratch, I'd say 1.3 meters deep for koi. They need depth not only for winter, but also for exercise and to feel safe from predators. On another forum, someone posted about making his pond shallower. His koi went into hiding as deep as they could get and stayed there. Though when they had a nice, deep hole to dive into, they were often near the surface.

The pond will stay warmer if you dig part of it below the frost line, and then the earth will support the walls where the pressure is the greatest.

Install a bottom drain! This is very important if you don't want your pond to turn into a maintenance nightmare. Koi and goldfish both have a relatively high throughput for their size, and a bottom drain makes keeping the pond free of waste and other debris much easier.

I'm not suggesting that you abandon your timber pond, but here's a very interesting and very easy above ground design. Just a few sheets of plywood, some steel cables, and a liner. The huge advantage of this design is that the cables take the weight of the water. Other above ground designs need to be much more heavily built.
http://www.koi-bito.com/forum/main-fo ... ine-tank-6.html#post99239

You might want to think about goldfish as an alternative to koi. They are available with similar color patterns and are even more suited to ponds. Do keep in mind their colors will change as they grow, though. If you're interested, I can find some advice I was given on how to predict which small fish will remain interesting when they grow up.

I'd stay away from "fancy" goldfish at least at first. These misshapen fish are much more delicate, and as they can barely swim, they are far too easy for predators to catch. Subunkins, comets, and standard goldfish are very hardy, though, and swim quite well.

Goldfish are only get about half as large as koi, but that means you can have four times as many of them. Some say that you should have 1000 gallons for for the first koi, and 250 gallons for every koi after that. Others say only 100 gallons for each additional koi, but these are big fish, and to keep more than a few of them in comfort, you need a huge pond. Don't believe people who tell you that, "koi will grow to the size your pond." What this really means is the koi's growth will be stunted if their pond isn't big enough.

There's a current thread on the anoxic system here with links to older threads.
http://www.koiphen.com/forums/showthread.php?t=94835

Unfortunately, much of the discussion is of rather poor quality. In order to make any sense of it, you first have to understand each filtration system mentioned and then ignore anything said about it by someone who clearly does not understand that system or who is exaggerating the potential problems in order to "win" the discussion. The current thread is better than most of the older ones, but there's still plenty of disinformation. Fortunately, one particpant, Dkoinut, converted his pond from aerobic to anoxic filtration. In one of the older koiphen threads he goes into detail about his experience and water quality tests.