If the summer weather is very hot and dry you may find that your pond loses a lot of water by evaporation. Topping up with tap water can cause problems. The nutrients in the tap water encourage the rapid growth of 'algal bloom' - a green slime. This looks unattractive, uses up a lot of oxygen and can cut out the sunlight too. Don't use chemicals to kill this 'blanket weed'. The best thing to do is to rake it out. Remove some of the other plants too, if they are threatening to take over. When you rake out plants, leave them lying by the pond edge for a day or two before disposing of them; this allows any trapped animals to crawl back into the water. Remember that having a large population of water snails is a great help - snails munch their way through much of the algae as it grows.
You will probably be amazed just how quickly a brand new pond matures into an established habitat. It is fascinating to watch your pond improve as time goes by and both you and the wildlife you are helping will derive much enjoyment from the new habitat you have created.
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