It is also important to treat the pond with enough chemical to knock the clay out of suspension. Too little chemical can sometimes just be a waste of time and money.

Is your pond roughly rectangular?

Estimated volume:

90 x 3 x 45 x 3 x 4' (avg. depth) = 145,800 cu. feet of water.

145,800 cu. ft x 7.48 gallons/cu. ft = 1,091,000 gallons. [You can fiddle with the dimensions a little if you want to consider some different volume estimates.]

If you test a know volume of pond water in your jar, you can determine the minimum amount of chemical required per gallon and then add a "safety" margin just to make sure.

I am NOT a pond treatment expert. Just throwing out some more ideas for you. You might come back with some more info just before you are ready to make an expensive purchase and hopefully the actual experts can provide answers to your final questions.