Koi may jump out of your pond and die.
Stephen M. Meyer
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. 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 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.