Nix on the larger UV, provide shade to the pond either directly or with
lilies, parrot feather etc. What is your plant situation like? More is
better. Lots of plants will use up the available nutrients in the water and
literally starve the algae. Remember when you feed the fish, you're really
adding plant fertilizer to the water, it just has to pass through the fish
first. My suggestion is to feed the fish minimally, they'll be fine, shade
the pond some and add more plants. Then just wait, maybe even a year. Oh,
your fish load is OK for now, but as the koi grow to size, you will need to
have fewer fish. 20 full size koi will be too many for a pond your size. (
Pause while I put my flame suit on ) Some would say 20 koi would be alright,
but you already have water quality issues. Koi really need 200+ gallons per
fish in a well balanced pond.
Just my $.02, feel free to spend it as you like,

Paul