According to this article many people are causing damage to the wildlife in their garden ponds because they don’t know how to properly maintain them. A survey of garden ponds found that many had high levels of nitrates usually caused by topping the pond up with tap water. Creatures such as water boatmen, beetles, snails, alderflies and damselfly larvae are adversely affected by high nitrate levels because they cause excessive nutrients in the water. This encourages plants such as duck weed and blanket weed which tend to suffocate the pond. Tempting as it might be to top your pond up with tap water when the water level is low it seems that most freshwater creatures are adapted to changing water levels. Most can apparently survive in just 2cm of water. The message here is to refill your pond with rainwater where possible although I can see that if you also have fish in the pond sometimes tap water might be the only option.