A clear Pond.Most of you have seen it before I guess: a fishpond that looks like green soup.... The filters can't handle the fine small particles that pass your system like there were no filter at all, and often people see no other way then to add all kinds of Pond cleaners which not just get rid of your floating green algae, but also affects your filter system and her eating organism.You should refresh at least 10% of your water/weekly. Check on your salt level (1g/ltr). Through the years your Koi grew bigger and for sure they're producing more 'pollution'...and of course the number of fish you got...!! The quality of the food you use might be a factor as well.
When you don't have an extra pump, you can use a net and fill it with pool wool/cotton and place it in the water stream.
My favorite however is a bucket with a dozen holes in it, also filled with some wool/lava rock or anything that absorbs small particles. The pump has a capacity of 6000 l/h so the holes have to be at least 1" or 2 cm. The more holes the better. It also depends on the polluted water and the kind of wool your use on how often you have to check on the holes for being clogged up.
I use two kinds of wool, the white one is the finest while the green has a more open structure to catch the bigger particles. Fold a layer of the white cotton and cover the holes, next: make a ball of the green cotton and fix the water outlet with a stone or something in the middle of this ball.
When the water reaches a higher level in the bucket, switch off pump and rinse/renew the wool.
If all of this doesn't work and the water is still greenish after this kind of filtering, there's only one option left: you need a (bigger) UV filter . I use a 30 Watt ProClear on a 3500l/hr water pump, which should be enough for Ponds up to 20.000 ltr. It's switched off during winter season and operational from around May when the need is there...
Within 3 days there'll be a significant change of water quality already, it will be crystal clear within a week....!
Remove filamentous algae longer than 10" by hand/stick rather than the use of additives which kill all of it....don't remove shorter algae.