bought house with koi (1) pond..long, sorry

Hi, I recently bought a 1923 bungalow in Atlanta, and I inherited a small
pond (about 4 feet across, maybe 2-3 feet deep) with one fish. To me he
appears to be a large goldfish, not a koi--is there any easy way to
differentiate? He's all orange. The owner that sold me the house said
that he was here 4 years ago when HE bought the house. The pond is chock
full of plants, and lots of green slimy stuff and some small lily pads. It
does have some sort of filter. I realize that I should probably clean it
out a bit, and I'm going to remove floating debris to start, but I'm kind of
thinking, "if it ain't broke, don't fix it." He's been here for years,
doing well (the former owner rarely fed him--I'm sure he's been chewing on
the plants and bugs) so I don't want to go in and clean out the pond and
shock his system. I would like to introduce a buddy for him though.
My problem is that I feel that I must give this little guy a good
environment, but at the same time I am overwhelmed with all the things that
need to be done to my "fixer-upper" so I don't have tons of time to devote
to this project. Like I said, he's been doing fine so far, but I would
like to make his life a little better. Advice?
Thanks in advance.
Kirsten