Newbie need help

I have a 25'x25'x24" pond with a 60'x2' stream. 40% covered in plants. 4-6
6 inch goldfish, and some baby goldfish, a few frogs. External self priming
pump and no skimmer or biological filter. No stones on the bottom of the
large pond. The water recirculates up to a 3'x3'x12" pool with stone in it.
The water then cascades over a 36" fall into the stream. The pond is
relatively clear.

1. Do I need to install an expensive man made biological filter? If so,
can I use small stones in the smaller pond to act as surface area for
bacteria?

2. Do I need to clean the bottom of the main pond on a yearly basis? If so,
why? Can I do damage to the natural system by stirring it all up?

3. Evaporation? Given the dimensions above and our summer temps in the
60's to 70's, I have noticed that the level of the large pond area has a
loss of about 1/8" for two days. We've had no rain. Given the surface area
of my larger pond area and the length and width of the stream, is there any
evaporation that I could actually measure?

Thanks for your help.

--
Steve Shapson
Southern Wisconsin