You'll want a combination of plants in order to properly filter your natural pool and support a healthy environment. Typically, you'll want some floating plants that rest upon the surface, some emergent plants whose roots are underneath the soil but reach up above the water, and underwater plants found underneath the water's surface. These naturally aerate the water, absorb chemicals that can be damaging, and provide food and shelter for aquatic life.
Look at your local streams and rivers to see what seems to grow the happiest there. This will make it likely that they'll thrive in your backyard too.
