It's a terrible pest.
Garden ponds can be really beautiful, adding a sense of peace to a garden like nothing else can do.
Others can provide you with a lot of food. The pond is an old bathtub propped on bricks. Sometimes we aerate it a bit with a small goldfish tank aerator, and we throw a handful of compost in it every month or so. All we put in it at first was a bathplug, water, and a couple of water hyacinths from a nearby paddyfield, which came with some duckweed. The tub quickly turned itself a balanced environment.