Partial water changes are an important part of regular maintenance. Removing 10 to 20% of the pond water and topping up with fresh not only replaces any water which may have begun to stagnate, it also dilutes any pathogens or toxins which may be present. Water changes should be carried out at least twice a year, normally at the beginning and towards the end of the season, but they shouldn’t need to be done more than once a month.

There is some debate as to the best way to source water when filling a pond or topping up after water changing.Many ‘professional’ pond keepers use distilled water, usually obtained for ponds by passing tap water through a reverse osmosis (RO) unit. The main problem with distilled water is that it is extremely soft and contains no or very few dissolved minerals. These minerals must be replaced to avoid causing the fish to suffer with mineral deficiency. Although the minerals can be added to an exact amount giving near perfect pond water, this process can become quite expensive and time consuming.Rain water is another option taken by some pond keepers, but rain water is also almost distilled in character and mineral additives will need to be used in the pond. Rain water may contain pollutants or dust out of the atmosphere, or which has washed off roofs.Natural waters from near by ponds, rivers or wells may contain pollutants or pathogens and, unless very confident of its quality, shouldn’t really be used.The simplest option is to use tap water, but this too has potential problems. Tap water is required by law to be fit to drink, not fit to keepfish in.There is a variety in the hardness of tap water across the country, with some areas having excessively hard water and others having tap water which is too soft for ideal pond conditions. There are also nitrates present in tap water, as well as chlorine and chloramines.Chlorine is highly toxic to living material and is added by water companies to sterilise the water. Chlorine does naturally diffuse from tap water. If tap water stands, in long pipes for instance, it may lose all the chlorine before it reaches its destination.Chloramines are essentially chlorine and ammonia molecules bonded together. Chloramines break down slowly, releasing chlorine gradually. When topping up with tap water it is essential that the chlorine, which will kill filter bacteria and may harm fish, is removed. This can be achieved by standing the water for 24 hours and letting the chlorine diffuse. This process can be sped up if the water is aerated. Chlorine, and more importantly chloramines, can also be removed with the use of a tap water conditioning solution.