Hi Sky!
Glad you found us!
I noted Solo gave you an answer on RP.
We have other folks here with larger ponds
who can answer also.

My thoughts - if it is stuffed full of fish, it might
need to be thinned out. Too many fish means
too many nutrients for the green water (suspended,
free floating algae cells) and might be using up
too much oxygen to keep them all alive.

Some more information would be helpful ~
how deep is the pond, is it uniformly deep?
is there electricity available to the pond?
what is your friend's goals for the pond?
is there any danger of runoff going into the pond?

k :-)
http://tinyurl.com/6bguh ~ new pond keeper info
http://tinyurl.com/yp64db ~ slide show of pond