I kept Aquarium Fish Magazine's from late 80's and early 90's. When I discovered a four part series on ponds. I went back and re-read them all as I started my first pond this year.

If you really want to calcualte ammonia production you can try to find the April 1990 issue of AFM.

Here is an example from the article. 1200 gallon pond, 42 koi 10 inches, each one is 190 grams (the artical has a koi inches to gram table to lookup the weight of koi). Each koi produces 47.5 milligrams of ammonia per day totalling 1995 milligrams per day. Dissoveled in 1200 gallons of water creates 0.44 ppm of ammonia each day.

The articele says do divide your pond size in gallons buy 125. The result is the required filter medium volume in cubic feet to have sufficient surface area to remove 100% of the ammonia produced. It also stated that 1 pond turn over per day is requried to use these calcuations but had a graph of pond volume buy surface area in cubic feet by pond turn over to help. if you turn over 1/2 the pond per day then you need more cubic feet of media.

It was a very detailed article on how to calculate ammonia production, volume of media, flow rate and disscussed filter designs.

Of course you could just wait and measure ammonia too and forget the math