Vicki,
There are many filter systems used by ponders but keep in mind it is a system. Since it is a run off pond with rocks on the bottom I don't think you have a bottom drain. So that in itself means eventually you will have to vaccuum the pond or drain and pump out the mulm and clay that accumulates under the rocks. Most people with rock lined ponds do that yearly. Pond skimmers will help keep the surface debris from eventually ending up in those rocks.

Bead filters and pressure filters usually require a prefilter before them, and you must frequently flush them wich requires a lot of water. May not be a bad option if you install a skimmer and use it after the skimmer. But you must commit to those flushes.

Your pond is twice the size of mine. How deep is it? What are the dimensions. If it was my pond and the theme was more natural vs crystal clear here is what I might do.

I might just use two large skimmers and take them to skippy filters. I don't think I would retrofit a bottom drain due to the clay. So I would do a yearly clean out. If the clay was not that big of a deal I would retrofit a BD to a settling pond or chamber and take that to a skippy.

Others with better ideas will chime in soon.

Regards,
Lisa

Lisa,
Thank you for the recommendations. My pond is 20 feet by 30 feet and 40 inches deep with some shallower areas. I need the mud in the bottom of the pond for the turtles in the winter so I did not put in a bottom pond drain. We have an overflow that feeds into a second pond for heavy rain.

I will look at skimmers and building a skippy. There are so many product choices and systems so I appreciate your help. Vicki

If turtles are your main concern you should join a turtle forum but you really don't need mud on the bottom of your pond for them to winter over sucessfully.

the two drs.have an oase bio tech 36,it is rated for 37000 gallons.i made one out of plywood lined with pond liner.it is three stage,first a 4x4ft. settle chamber, overflow to a 2x2ft.blacknight brushes,then goes under and up to fine filter pads to bio or you could put the bio in a w aterfall.i would add a skimmer. turtles are difficult to keep and they tear up any plants in the pond.a good energy efficient pump is a performance pro.good luck

If you have lots of surface runoff it could give you lots of water clarity issues. Here is a good turtle forum if you are interested.http://www.turtsntorts.com/tntforum/