i don't purge before harvesting, and haven't had any off flavors,, some "pond" fish can taste muddy, and some off tastes are associated with bacteria in the water that the fish were in..not usually an issue with AP

you shouldn't have to do any water changes if you keep your system in balance - that is, stock to your filtration capacity, and feed accordingly.. if you are getting a buildup of solids, you are either feeding too much, overstocked, or underfiltered