I started out in a similar position in may/june of this year. I went
from 200 gallons to around 1350 gallons when I upgraded my pond. At
first I had about 10% coverage with plants, plus my purifalls was
filled with WH. My water turned to pea soup for about 2 weeks, then
only cleared to about 1 1/2 foot deep(The pond is 3 ft deep). During
the day the algae got worse when in full sun, after dark it cleared
some. I broke down and bought a UV Clarifier, installed it and it
cleared overnight. I have a nice covering of hair algae now that is
about 1/4 to 1/2 inches tall around almost all of the pond. I do get
some longer hair algae where there is no shade during the day. BTW, I
am about to 50 - 55% plant coverage now. My UV cost $106 -- shipped
to my house and I am really happy with it. Also, I don't use any
chemicals in my water.

On another note, I also maintain a reef aquarium. UV is one of the
best things I have seen for keeping the reef in top shape -- of course
the UV is not only clarifying the water, but sterilizing it of nasty
parasites that are waterborn. If I ever get into any more expensive
Koi I will get a UV that can sterilize the water to help protect the
investment.