Is there a pump for the waterfall?
If so, where is the pump located in the pond?
Koi are just like other fish, they need variety in their diet.
I use Kensfish.com for my source of koi food.
From the picture you posted it looks like
goldfish not koi.
The first step for cleaning would be to remove all the plants, it looks like they are probably potted.
Place the fish in a large bucket so they are out of harms way while you clean.
Is there a substrate on the pond bottom?
If not, scoop out all the gunk off the bottom of the pond.
Is there any type of filtering system on the pond?