Fish hatchery's like Zetts sell fertilizer for ponds. You could try your local hatchery to see if they have any and what they advise.
I am no expert, but in a similar position as you. Newly reworked .5 acre pond in the north. Mine is still slowly filling, but I have had multiple blooms in the pond so far. Right now it looks like peanut butter after the hard rain a couple days ago, but it's starting to settle out.
Weeks ago after the first bloom the pond got pretty clear. I could see the bottom in 4' of water. Plenty of life in the pond so I stocked some FHM and GSH. We'd get a rain and the pond would cloud up a little and then turn pretty clear in a day or so. I added some alfalfa hay flakes around the edge of the pond to provide some cover and hopefully spawning habitat for the shiners. There wasn't any plants or grass yet as it was only 1/4 full. A few days after adding the hay the water turned green, next day it was even greener, then it slowly turned a brownish green color. Settled in on that nice olive green color and stayed there for a couple weeks until this last heavy rain. Clarity was down to 14"-18" and no surface growth or films that I saw. So the Hay must have added the nutrients the pond needed to get the good blooms. Long term I do not know.
I have 25-30 acres of hay fields that drain to the pond. No runoff has made it to the pond yet since it's been dug as we've been very dry and whatever rain comes down it is absorbed. I do use fertilizer and lime on those fields when needed, so I'll keep and eye on things.
