Let me address some of the most common ones. Most health problems are related to water quality or other environmental factors, which can be avoided by proper care and pond husbandry. This commonly happens when stocking levels exceed the owner's knowledge and husbandry skills. As stocking levels increase so does the potential for problems, which can arise suddenly with devastating consequences, often resulting in large fish kills. So the first stage is to he honest and realistic about your ability to manage a heavily stocked pond.
Fish often rub against solid objects and leap out of the water.
It is possible that they do this to catch flies or get food or they may be playing. However, in the majority of cases it is because something is irritating them. Whether you should act or not depends on the number affected, the frequency of rubbing and leaping, and the vigour involved. If one or two are having a lazy rub on the bottom drain there is no cause for concern, this is fairly normal. But if they are continually tearing about, rubbing vigorously against the sides or bottom or leaping out of the water, then further investigation must he carried out.