help needed please - just inherited a koi pond

hi everyone,

I just got a new house that happened to come with a koi pond, and I
know nothing. I'd really appreciate a few tips to get me started.

The pond has 4 or 5 fish, and I've not done anything to it for a month
and they all still seem to be fine.

1. do I need to feed my koi fish?
2. the water level has gone down due to the heat. Can I just refill
it with tap water?
3. it's really quite green.. is there a way to solve this and make it
more clear?
4. is there anything else I need to know?

thanks heaps!

- Brendan