The production process of an APU is an optimisation of the 200kg stocking mass that the unit can hold. You run them in monthly cohorts, or groups of fish, worked out backwards from the target size you want to grow your fish to.
By way of example, if you want to harvest fish at an average weight of 300 grams each and by knowing the growth rates of tilapia a simple calculation can be arrived at that looks something like this.
Day 1. Stock 350 fish at fingerling size of 8g each – Cohort 1 – total mass 2.8kg
Day 31. Add 350 fingerlings at 8g each – Cohort 2 – total mass 2.8kg. Cohort 1 will now be 34g each or 11.9kg.
The final equation will look like this
Cohort 1 = 350 fish @ 8g ea. = 2.8kg
Cohort 2 = 350 fish @34g ea. = 11.9kg
Cohort 3 = 350 fish @ 84g ea. = 29.4kg
Cohort 4 = 350 fish @ 165g ea. = 57.8kg
Cohort 5 = 350 fish @300g ea. = 105kg
Total fish mass in pond = 207kg
Hence every 30 days you will be taking 350 fish at 300g out of the system and adding 305 fish at 8g to replace them. This means you are harvesting 105kg per month per tank for a total of just around 1,200 kg per year.
Of course this is when the system is hitting its straps and you are in a production rhythm that allows you to maintain these stocking densities and growth rates – it is learning to get this cadence of production rhythmic that is the difficult skill to master.