I work seasonally at a Renaissance fair/event venue in Oklahoma. We have a large (50ft-150ft diameter?) man made pond. The pond is roughly 15 years old, maybe 20. It has a fountain in the middle and local turtles and frogs have established themselves in it, but to my knowledge it has no fish. The water is currently murky and the pond as a whole is very drab. I don't have any pictures at the moment but intend to get some this weekend.

My main question is this: Are there plants, substrates, bacteria, or creatures that we can add to the pond that will improve it's water quality or at least make it more aesthetically pleasing, without negatively effecting it's current turtle and frog inhabitants?

The owner is a penny pincher, so a filtration system simply wouldn't be an option, but he loves nature so he would be willing to add things to it to create a natural bio cycle so the pond would be self-maintaining. The space has so much un-utilized potential, and I would love to see it improved.