It sounds like a combination of LOW-LIGHT algae and a bacterial coating. I
had something similar in a fish tank some years ago. It was solved by
increasing the amount of light and more partial water changes. You may
also need a bigger filter. You may have too many fish in your pond. What
about plant filtration? Do you have plants? You may be feeding them a
little too much for the amount of *partial* water changes you're doing.
--
McKoi.... the frugal ponder...

Thanks for the comments, and Yes, I do have very low light for the pond, I
only turn on the 160 watts light when I want to see the fish, it's in the
basement, so I only run a 23 watt energy saving light when nobody is in the
basement. I have 30 cichlids range from 4" to 12", my filter is 2400GPH
wet/dry, my nitrite and ammonia is always 0 though, no plants, change 1/3
water every week, and feed them 3 times a day as much as they can consume in
5 minutes.

I will turn the primary light on for a few week and see if it improves.

DJ