Yes, you can use tap water in your pond, however the important thing to remember is to dose the new added water with an appropriate volume of water dechlorinator. This chemical is readily available from pond shops often labelled as “tap water treatment”, this chemical will detoxify chlorine, chloramine and heavy metals which all are harmful to aquatic life after long exposure. A question often asked is can I use rainwater? The problem with rainwater is it is too soft; it lacks any substantial volume of minerals which are essential for bodily functions in Fish. Long term use of rainwater often leads to a crash in parameters of the pond leading to mass fish loss. Whereas tap water, especially in the southeast of England, the water is very high with a high volume of minerals perfect for long term sustainability in fish health.