Q. I am going to put a koi pond in my yard. I have read about the subject, but am wondering what kind of pump to buy. I have experience building a small goldfish pond without a filter system, but I know that koi need some kind of filter and running water to be at their best. Could you suggest what to buy and where in my pond to install it? Also, will koi and goldfish live well together?

A. Let me begin by clearing up a very common misunderstanding about koi, koi ponds, and pond filter systems. Koi are just fish — like any other fish — though they can be pricey. In fact, they are carp, so they are substantially less finicky about water quality than many other types of fish.

So why, you may ask, do all the books and hobbyist articles emphasize the need for special koi pond filters? The answer has nothing to do with koi, but with koi keepers. Koi ponds tend to be packed with fish, far beyond any reasonable limit. Small wonder, then, that koi ponds need special filtration systems (which only help partially).

The need for biological filters in ponds is, as in aquariums, a simple function of fish load — the mass of fish per unit volume of water. It does not matter if you are keeping koi, goldfish, cichlids or trout. If you keep the fish load low you can avoid the need for a pond biofilter entirely.

In any case, I am a strong advocate of recirculating water in ponds — even in ponds without filters. Specifically, recirculating water helps to increase oxygen concentrations in the water and remove harmful gases. All fish species are healthier and look better under such conditions. Therefore, a pump is a good idea.

To properly size the pump, consider that you will want to recirculate the pond's entire volume every two hours. So, if your pond holds 3000 gallons of water the pump should be able to move at least 1500 gallons per hour exiting the plumbing. The qualifier — exiting the plumbing — is often missed by pondkeepers. Maximum flow rates for most pumps are usually pegged to 1 foot of "head" — that is, the amount of water that will exit from a 1-foot-long vertical pipe attached to the pump outlet. (Head is a measure of resistance to water flow due to gravity and friction along the plumbing path.) This is of no consequence in standard aquariums because the piping distances are very short and the flow rates are very high. But in ponds, flow resistance can make a tremendous difference in the actual recirculation rate.

Most pond plumbing systems pose at least 5 to 10 feet of head; some may be many times this. Thus, when buying a pump it is important to purchase one that has the flow you require at the head resistance of your piping system. If you are not sure what that head resistance is, you may assume it is between 5 and 10 feet and buy a pump on the condition that you can return it if the direct measurement shows it does not meet your needs. Put a bucket of a given gallonage at the end of the pipe, start the pump and time the number of seconds required to fill the bucket. Divide 360 by the number of seconds you timed and multiply that figure by the gallonage to fill the bucket — the answer is gallons per hour.

Make sure you buy a pump suited for continuous duty. Many people make the mistake of buying a cheap sump pump only to discover it burns out in three months of continuous use.

Next, consider power consumption. Look at the wattage rating for the pump. From among those pumps that provide the right flow rate at the head resistance of your pond plumbing buy the pump with the smallest wattage value. (If wattage is not shown, buy the pump with the smallest amp value. You can estimate watts by multiplying the amp value by 110.) Horsepower does not tell you anything useful.

Does it matter if the pump is submersible or external to the pond? Not in my experience. I use both. Submersibles are noiseless, which is a big plus. Feel free to put the submersible in the pond, away from the water return inflow, and be sure to install a ground fault receptacle.

Now, about koi and goldfish. It is not a good idea to mix these fish. They will certainly get along well — too well. At breeding time koi and goldfish mix it up, with the goldfish always suffering serious bruising from the affections of their much larger cousins. True, my pond comets have held their own, but always with lots of bumps, bangs and scrapes. In particular, fancy goldfish get clobbered, if they don't starve to death first — the koi beat them to the food every time.

There They Go
Q. I have maintained aquariums for 11 years and have had great success. Ponds and koi are a different story. I built a 1000- to 1200-gallon, formal above-ground pond in April 1994. I introduced 25 koi about 4 inches in length each. The pond had two Tetra G pond filters, a fountain with a filter on the pump, and several types of floating plants. I do 20-percent water changes weekly. I have two questions.

First, during the summer of 1994 five koi died. Two committed suicide by jumping out of the water. The others had no obvious symptoms after an external examination. Upon internal examination, however, I found their stomachs or intestinal tracts were twisted. It appeared that half of the stomach had been turned upside down, with part full of food and the rest of the tract empty. What caused this?

Second, living in Colorado and having an above-ground pond, I decided to move the koi indoors for the winter. I purchased a 135-gallon horse trough and placed it under the kitchen table (made my wife really happy). I attached two canister filters, two bio-wheel filters and one Tetra G sponge filter to it. In October of 1994 I moved the koi indoors.

The tank cycled great. I had zero ammonia, zero nitrite, zero nitrate and a pH of 7.0. I lost six koi that jumped out, so I installed a glass top. In the fourth month (January 1995) nine koi died in a 24-hour period. I cannot find anyone who can pinpoint what happened. The fish did not have any internal or external symptoms. I determined that a toxin had gotten into the tank. In desperation, I did a 100-percent water change and tank scrub down. I did find a small piece of lead, about the size of a little fingernail, and as thick as aluminum foil. There was also white slime dripping out of the biowheels. Could this kill the fish? Might it be the coating on the horse trough?

A. Well, my assessment of you situation is not going to make you happy. First, koi do jump. They jump for fun, for exercise, for who knows what.

If your fish are systematically jumping out the water it is because they do not want to stay there. Usually this is caused by intolerable water conditions: too low or too high pH, irritating concentrations of ammonia or nitrite, too low oxygen concentrations, or chlorine in the water.

Koi do not have stomachs — they have long, winding, twisted, looped intestinal tracts. So what you observed in your koi was a normal digestive tract layout.

I am absolutely certain that your winter setup deaths had nothing to do with the lead flake you found in the tank, nor with material or coating on the horse trough (which is made of a PVC plastic and is perfectly safe). Nor is it some mysterious toxin. The white slime you describe could be several things: mineral deposits leaching out of the water or concentrated dissolved organic substances from fish slime and food wastes.

Given what you have told me, the problem is almost certainly low oxygen levels in the water. You have too many fish in too small a volume of water, with too little oxygen. Larger koi tend to succumb to low oxygen sooner than small koi. This may be an intermittent problem — for example, one that occurs after a heavy feeding — rather than a chronic problem. The large number of sudden deaths within a short period of time supports this assessment.

I assume your water does not contain chlorine. If it does, and you do not treat it properly, chronic chlorine exposure could explain all your problems.

You did not say if you regularly measure pH and nitrogen pollutant levels. A high-capacity biological filter serving a very high fish load (as you had in the tank) will lower the pH significantly over time. Koi do not care for a pH under 6.5. With indoor holding tanks you should routinely measure water quality each week.

Had you started out with 10 fish in the indoor tank, they all would have made it. What you are watching is a biological system degenerating to carrying capacity. Mother Nature solves the problem of too many fish in too little water under stressful conditions by killing some of the fish. And, of course, those fish are almost always the favorites or most expensive animals.