When I bought my house last year it came with a small garden pond well-stocked (in fact probably a bit overstocked) with fish - some quite large carp, goldfish, one or two koi. It has a series of small upper ponds and the water circulates in that way. We had the pond cleaned last summer and have regularly cleaned the filter, etc and it has seemed to be OK. The odd fish dies from time to time but overall no apparent epidemics etc. However we've noticed recently that the fish all seem to be lined up with their noses by the small waterfall which discharges into their pond. Is this a sign of something wrong? Lack of oxygen? We are complete beginners.
The problem is water quality. I had the exact same problem in a pond in Mexico. One of my water pumps, a small one, stopped working. Time passed and the fish grew. As the pump was small, I did not replace it. About two or three months later some of my fish began to die, especially the smaller ones without showing visible signs of being sick. 1 or 2 every two or three days. A fish dying from time to time is normal, but three or four fish a week is no normal. The fish continued to eat regularly and did not look sick or act strange or different. The only new thing was that most of them congregated more often under the waterfall, sometimes for several hours and seemed to struggle or strive to have a privileged place in the jet of water. Beside this, there was no new variable implemented in the pond.
The new behavior of the fish was that much of the day, but especially, in the mornings, most of the fish congregated under the waterfall, at first it was only in the morning, but as the days passed, they congregated under the waterfall for longer, sometimes almost all day. The only thing that interested them was being under the waterfall. The fish didn't seem to be looking for oxygen, rather they were looking to clean themselves, as if something in the water was irritating them and they were instinctively looking for new water to clean or remove the irritation. Obviously, the fish do not know that the water they received was not new, as if it could happen with a waterfall in nature. The pond is a closed circuit with the same water.
How was the problem fixed? I simply incorporated a new pump, which improved the amount of filtered water and generated greater oxygenation in the water. Two days later, the fish stopped congregating under the waterfall and stopped dying.
If the fish congregate below the waterfall it is NOT normal, it is because they feel irritated or bothered by some harmful component in the water. It could be some bacteria, excess debris or some chemical compound that is not being properly decomposed by the filtration processes of the pond filter. If the fish congregate under the waterfall it is a sign that they are bothered by something present in the water and it will be necessary to analyze in each case whether greater filtration, greater water flow, greater oxygenation is needed or we have an over-population of fish for the amount of water. Or maybe it's all of the above. The most important thing is to detect it and act before the fish start to die. If your fish congregate under the waterfall, it is because they are uncomfortable with the quality of the water. As simple as that!