The water within your pond may appear to be clear and therefore presumably healthy, the fish may look healthy and the plants are growing well but the reality may be far different to what we can see with just our eyes. Far to often a pond will be built in a distant corner of the garden and therefore can be subject to neglect as it is out of sight and out of mind, especially during the winter months .
To ensure that your pond life has the conditions that are required for a healthy life you need to pay a little attention to the quality of the water, which after all, forms the boundaries of their world. It is often said that if you look after the water then the fish will look after themselves and ignoring hereditary diseases this is generally true.
You should be aware that pollutants which find their way into or naturally occur within a pond (such as Dust, Leaves, Grass clippings, bird and fish droppings) will rot and dissolve into the pond water. When the water evaporates and the pond level drops the toxic pollutants do not evaporate and over time will reach dangerous levels even if water top ups are done regularly.
Domestic tap water although suitable for human usage, is not immediately suitable for fish; this is because your local water supply company must add chemicals such as chlorine, which destroys bacteria. Tap water also contains heavy metal trace elements (iron, copper etc.), which are also toxic to fish. These additions to the water will irritate the fish, and can even cause fish deaths. Do not use water from other sources such as rivers or wells to fill your pond.
For these reasons you should always add a good quality pond dechlorinator liquid to your pond when adding tap water and just topping up the level is not adequate as you should perform regular water changes (remove up to 25% of the ponds water and replace with treated tap water). Changing more than 25% may cause unnecessary stress on your livestock because it can cause large changes in the chemistry of the water such as the pH value.
If possible try to avoid making water changes at the weekend as this is when the local water boards tend to automate the chlorinating process which can result in a higher level of chlorine than during the week when the process is monitored and regulated manually.
Check your pond water regularly for ammonia, nitrite, and pH etc., these are the first signs of an oncoming problem and are very simple and cheap to do with the easy to use test kits, which are readily available, at all good pond retailers. If these test kits show a problem you should perform a series of water changes until the levels have reduced to safe parameters.