HI,
IMO stands for In my opinion. IME would stand for in my experience.
Ok, You have a lovely sized pond. We swear by good filtration for our pond with a UV light built in (not just a pump etc) and bought one over sized for the pond capacity to keep the water free from green and with clear water all year round.
Yes, it would be greener at this time of year - the winter murky look!
Yes, a hot sunny location would encourage more green to grow and providing shade is also a good idea, as already recommended. Koi will nibble plant leaves. They are known for pulling plants out of their pots I'm afraid, just as they start to grow. They tend to leave things alone one they plant gets quite big.
Another suggestion is to start planting up the outside of the pond to provide shade. We have had to do this as our pond is in a south facing garden and without the plants boy it would be too hot for them.
We have used tall grasses and phormiums and as the sun moves they cast shadows in the pond, enough to provide shelter. The shady areas also grow less green algae?
Our pond photos are in the gallery if you want some kind of idea.
There are some pretty variegated pond plants (marginals) that could be kept in pots just below the water surface, (so the fish can't get in the baskets to eat anything) and they would grow across the pond. If they go wild, just pull what you don't want back off the pond? They all die back in the winter anyway.