Welcome to the forum!
I may be off base, but I'd hold off on putting any fertilizer in there. I think you have plenty with the nutrients from the horses. Check your alkalinity when you get your test results back. You might need to lime, depending on the numbers.
Look into Tilapia to keep the Algae under control, and give TP&W a call and see if Triploid Grass Carp would be an option for the other submerged vascular weeds. They like APW.
I think that if you can keep the FA from growing, and knock back some of the weeds, that will free up a lot of nutrients for a bloom.
Check in the archives, there is a relative weight chart for fish. Utilize that to check the growth rates of the fish in the pond.
