There is no doubt that having standing water in a water lily attraction will invite mosquitoes. If you keep goldfish in your pond they will eliminate mosquito larva, and if you are simply using the water as a display for your water lilies you should invest in biological controls known as mosquito dunks, which look like donuts. These get tied to the side of the pool and will keep mosquitoes at bay for a month. It is also important to remember that your water lily pond is likely to attract dragonflies, which eat mosquitoes.