Last year I installed a small backyard pond that holds about 1,300 gallons. It is only a foot deep, which is fine for our area because it never gets cold enough to freeze. I keep a dozen or so fancy goldfish and three medium-size koi in the pond.
My question involves feeding. This past spring my fish ate like crazy. They couldn’t get enough food. I must have been out there eight times a day dumping pellets in the water (making sure that they finished everything within 10 minutes). Now, they barely eat all the food at just two feedings a day. I am worried that they are sick. Because it is so much warmer now, I figured they would be eating even more! A friend who raises tropical fish suggested they may have an internal infection and that I should treat with an antibiotic. Any help you can offer would be greatly appreciated.
Hold off on the antibiotics, Bob. You would be wasting your money and endangering your fish.
The first thing to consider in changed eating behavior is water quality. If the fish were eating like crazy and suddenly have cut back, there could be a problem with pH, dissolved oxygen, ammonia or nitrite. Test the water immediately.
Why would the problem crop up now? As the water warms and nitrification accelerates, so does the acidification of the water. If your fish were eating as much as you say, the nitrification could have significantly lowered the water’s pH. Once it gets below 6.5 or so the fish may not feel so healthy or hungry.
Higher air and water temperatures mean lower concentrations of dissolved oxygen in the water. Feeding fish consume much more oxygen per minute than inactive fish. If oxygen levels are below 5 parts per million (ppm) consider additional aeration via a fountain, a waterfall or a bubble aerator.
The feeding frenzy earlier in the year may have corresponded to a growth surge in the fish, and now you simply have too much fish waste (and excess food) going into the water. In other words, the fish load is much larger than six months ago. (Because you didn’t mention anything about pond filtration, I can only guess that you may not have adequate biological filtration.) If you do have measurable quantities of ammonia or nitrite present, a 50-percent water change is in order. You should then use an ammonia remover, or add 1 pound of salt per 200 gallons of water, to mitigate the nitrite poisoning. A more permanent solution will require a pond filter — or fewer fish.
Of course, it may well be none of the above. Pond fish eat excessively in the spring because they go on a starvation diet during the winter (even in your area). They may lose 20 percent or more of body weight during the winter.
When spring comes and breeding season approaches, they start to pack in the food. So, what you saw in the spring may have been a seasonal anomaly. As the weather warms and breeding season passes, goldfish and koi cut back on feeding. I notice this clearly every year in my fish.
Then, too, by mid summer algae and other natural pond foods become available and so the fish eat even when pellets are not available. If all the fish are feeding and they act healthy, then you are probably just observing a natural cycle. As you gain more experience with the fish these things will become second nature to you.