Keeping a good, productive pond is a constant battle against natural processes that keep habitats in an ever-changing state. If given enough time without intervention, they fill up with silt, become shallow, then turn into a bog, marsh and finally dry land. But the process can be postponed indefinitely. Regular observations, wise assessment of its condition and careful management can keep ponds pretty and productive for years. Ornamental, decorative ponds that are also recreational fish ponds require the most work.
Actual maintenance is periodic and done when changing conditions require it.
Knowing when to make a change requires monitoring such things as aquatic weeds and fish populations. If you know what is normal and what to expect in your pond, you are then able to recognize a problem when it occurs. Problems with the fish population are evidenced by changes in the fish. If their size is decreasing but more are being caught, there are likely too many fish in the pond. Sometimes undesirable fish numbers increase and pressure other populations.