Do you know what is in your fresh water? As in do they use chlorine
only or are there non-evaporating chemicals as well? If it is just
chlorine, you need only circulate long enough to get rid of it. A
couple of days would be more than ample if you have good circulation.
If you have other stuff, you will need chemicals to get rid of them.
Others will have advise you there...here in MS we only have chlorine.
We top up with a toilet valve, so the amount is small enough that we
can just ignore it.

When you reintroduce your fish, be sure to let them adjust to the new
water for temp and composition. You will want to mix waters so they
don't have a shock going into your new water. There have been posts
on that subject and I suspect some others will be able to advise you
there. We have used an add 1/4 of the water every 1/2 hour rule and
done the mix five times. That takes 3 hours. By adding 1/4 of the
volume each time (and having 1/4 run out of the container), you have
only about 1/3 of the old water left after 5 times
(100,80,64,51,40,32). Then we net the fish and put them in the pond.
The temps are the same and the netting means essentially none of the
water they have been in goes to the pond.

On the subject of green water: It comes from nutrients in the water
that the algae are using. It is likely to return if yu don't have a
good system to remove the nutrients. It tends to come at the start of
the year before the plants get going. Algae cannot compete with
vascular plants, so it fades once they are under way...if there are
enough of them. Lots of people use a UV filter for the green times.
Our pond went green annually for perhaps five years. We used a UV
until the plants took off. The last several, it has not had green
times at all. Strong plant growth. We do get string algae that begin
to 'rot' once the plants get going. Our koi view string algae as a
treat, so it is never in the main pond. It grows in the veggie filter
and the water falls.

Jim