When is the ideal time to "purge" your grow out pond into your larger pond???
What species do you have in the growout pond?
IMO I would 'purge' in the fall and ideally allow the growout pond to dry during the late fall and winter with a refill in spring. Drying allows better and faster decomposition of bottom sludge and helps control (reduce) many aquatic weeds.
Coppernose bluegill
Smallmouth
Yellow perch
All have been in there or 3+ years
Please try and get some pictures of the nicer 3 yr old ones.