Hi,

How large/deep is this pond, and is it a pre-formed liner, flexible
liner or something else? Do you already have plants in there? Is there
gravel/rocks on the bottom or is it clear?

My first reaction is to do some partial water changes if you haven't
already. That probably won't be enough, but it will help. If you're
using city tap water, you may need to add some Aqua-safe or the like
to neutralize the chlorine. Depending on your setup, you can either
overflow the pond to replace water, or siphon some water out, ideally
from the bottom, and add new. You can use a garden hose for a cheap
but very slow siphon or get a 1.5-2" flexible plastic sump hose like
the sell at H-Depot for a faster siphon for a few dollars more. Of
course, that all assumes you have a lower spot than the pond that you
can run the siphon hose to and you are prepared for the possibility of
getting a mouth or face full of pond water. (Been there, done that.)

If the pond is really messy, and it isn't too large, you might want to
drain it and do a big cleaning, shop-vac style. You'd have to move the
fish to a kiddie pool or something like that and keep the water
quality fairly consistent (when they're in the pool and when they're
returned).

You really need some kind of filter running, though. Goldfish are very
messy. There may be some low-cost home-built solutions that list
members can help you with if they knew the size/construction of the
pond.

Dave