My first guess would be that they are lethargic because the water is cold. Staying on the bottom gives them several advantages. Down there, they are closest to the heat source (the ground under the pond). Water below 4?C will rise to the surface, hence when it's very cold, the warm water is in the bottom of the pond. And staying low helps sluggish fish avoid predators.
If they are just cold, all you have to do is wait for them to warm up. Feed them only foods rated for cold weather (wheat germ), until the water warms up, and if they don't want to eat, don't feed them at all. It's quite normal for them not to eat below 10 or 12 ?C, and many recommend not feeding at all until the water reaches that temperature.
I'm not sure about their tails. Please post some pics.
Peter, having taken over a badly designed pond with leaks and other problems a few years ago, I understand what you're going through. I seriously thought about filling the thing in a few times, but I'm glad I didn't.
Phosphate is said to be limiting nutrient of algae in most ponds. If you don't have a phosphate test in your kit, you should probably get one.
Does your pond get any shade?
How bad is that leak at the UV clarifier? If you're constantly topping off the pond, it's going to be very hard to get the ecosystem functioning properly.
Half an inch of koi per square foot (about 14cm/m?) seems to be a respected rule of thumb. Yes, this is rather conservative, but koi are big fish, and if you stock conservatively, you'll have overkill on your side if the summer is unusually hot or circumstances force you to neglect the pond for a while. The ideal formula takes into account that things are not always ideal. Here's a good article on stocking formulas. http://www.buildagardenpond.com/articles/howmanypondfish.html
If you run a large trickle filter and keep the pond clean you could stock more heavily, but what happens if the power goes out? I know it's hard to resist loading up the pond, but it's better to have more pond than you need than more fish than your pond can handle.
Re: What to do with the fish whilst fixing a leaking pond?
You could also build them a round liner pond. Just bolt together few sheets 1/4" of plywood, wrap the resulting frame with steel cables, and drop in a liner. This might cost a little more than the kiddie pool, but you'd be less in a rush to get the repair done, and hence might have less trouble with it. http://www.koi-bito.com/forum/main-fo ... ine-tank-6.html#post99239
Goldfish get larger than most people realize. In a healthy environment, they'll grow to about 30cm. Hence, a 30-40cm wide pond would be extremely cramped for them. Minnows are interesting to watch though, and there are many species that do well in narrow spaces like watering troughs.
Would the pond need to be that narrow over it's entire length? Perhaps you could dig an irregularly shaped hole and put a pond liner in it. The other advantage of this is that the earth would insulate and warm the pond in the winter.
Pumping in water from your old pond is a great idea, but I suggest that you seed the water in the new pond with a bacterial additive as well. I use Microbe Lift "Professional Blend" in the warmer months, but there are many similar products.
Another thing you should consider adding is barley straw. This will decompose and release substances which inhibit algae. The amount of straw depends on the surface area of the pond. For a new pond, try 50 g/m?. Half that will probably do once the pond is established. Put the straw in a nylon bag so you can easily remove it when it's used up. Each load of barley will last about six months. Because this method depends on bacteria to decompose the barley, you might want to use barley extract for the first couple months. Here is a good article for you to read. http://www.fishpondinfo.com/plants/barley.htm
Actually, by that formula 1600 gallons is room enough for 7 fully grown koi. This is fairly conservative, and you should check out other methods determining how many fish you can have. But if you can avoid overstocking, you'll have fewer problems.
I really hate to say this, especially if you're well into the pond project, but 900 gallons isn't enough for 13 koi. One rule of thumb is 1000 gallons for the first koi, and 100 for each additional koi. Keep in mind that these fish grow to two or three feet long. They also need at least four feet of depth to be comfortable. People claim koi will "grow to the size of the pond", but it seems to me that if any animal's growth is stunted by the size of its quarters at any stage in its development, it is being abused. Now, with aggressive filtration you could stock more heavily, but 13 adult Koi are too many for a 900 gallon pond. That's like trying to raise goldfish in a one gallon bowl.
And no, I'm not a big pond snob. I have an 1100 gallon pond just over three feet deep, and I won't have koi or orfe becuase I just don't have enough pond for them.