NEED SOME POND ADVICE

Hello,
I recently moved into a new place that includes a large pond on
the property (I'd say it is approximately 15 ft by 25 ft and deep), and
there are maybe a couple hundred goldfish in there. I have been feeding
the goldfish flake food twice a day, and they seem to be doing fine. I
have not measured the pond, but it seems to be fairly deep. There is a
lot of vegetation in the pond (lilly pads and other plants). The fish
seem to be doing fine, but I've been told I should have some sort of
filter or something on the pond. The problem is, I have NO electricity
on this particular area of the lot where the pond is. The water is also
a greenish color, so there is obviously algae in the water. Is this bad
for the fish? I know I can get some algecide to put in there, but would
that reduce the oxygen in the water? Is it possible to buy some kind of
"air" pump that just floats on the surface of the water and shoots air
down through a hose or something? Please advise me what to do as I want
to take proper care of the fish, but I don't want to spend a ton of
money, and remember there is no electricity at this location.

Thank you in advance. Any suggestions appreciated.