I have a database of fish hatchery data. I have an initial pond inventory and monthly mortality counts. I can easily get access to subtract the first month of mortality to provide an updated inventory but how do I do it for subsequent months for each pond?  

Example: Pond 1 Inventory: 70,000 in June 80 fish died. New July inventory 69,920 but 49 fish die in July bringing the inventory to 69,871 and so on