I have a pond I just got started about 6 weeks ago. 450
gallons. I have 5 Goldfish and 3 Koi in the pond. 1 Iris
running a 750 gph pump with fountain and a waterfall.
My water is turning very green and murkey. I put a clean filter near my
waterfall to filter it before it drops into the pond. In 1 hour the
filter turned dark green.

Filter isn't big enough.

Do I need to do water changes?

New pond, you should be doing 10% every 5-7 days and using a dechlor
product that also binds ammonia. Amquell, Ammo-Lock, ChorAm-X are some
brands.

Will this green water harm my fish?

It is what is keeping your fish alive, if not for it, they would perish in
their own waste.

Any suggestions about how to get rid of this
green algae?

Bigger filter, and as others have said, more plants. Water changes are a
must, frequent and small till the cycle is established, imho. Don't feed
the fish till the cycle is complete, and then start off with small feedings
1/day. ~ jan

See my ponds and filter design:
www.jjspond.us

~Keep 'em Wet!~
Tri-Cities WA Zone 7a
To e-mail see website