Water Gardening: Unlined pond / bioretention in clay, need advice to finish , 1 by Analemma

Analemma wrote:
Last fall I managed to dig this pond/ retention basin to collect the rainwater from our roof. We have non-draining clay soil. I have put in an overflow that leads to a ditch, and all winter the pond stayed full without adding water. The plan is to put in a waterfall, since I have a "mountain" next to the pond. But I am confused about what we need. I'm not planning on keeping koi in it, just some plants and probably some "throwaway" goldfish. I do want it to stay reasonably clean so the dog doesn't smell when she wades in. Right now, the bottom is covered with rotting leaves because we barely were able to finish the digging before the weather was unamenable in the fall.

The pond is roughly oval, about 14 X 20, and only about 18 inches deep average. The top of the "mountain" is about four feet above the surface of the water. Most of the filters I have seen are very expensive, and I wonder what I can get away with, since I'm not planning for fish. This is mostly a retention pond to handle drainage issues and gutter overflow, and to look pretty, and for the grandkids and dog to play.

Brenda