When frying at home .... I'm running the temp up to 325deg before putting the fillets in. That's in a iron skillet with peanut oil. Couple of minutes on each side and they're on my plate !!! And I'm talking 5-6 fillets from 10-12" fish at one time. The temp will drop back down to around 300deg, when the fillets are added, but climbs back up to near 325deg by the time that side is done.
I think many people overcook fish ... some because they like them crispy/crunchy, some because of the heavy breading they use, and some because they just don't know any better or out of fear of undercooked fish. It don't take long to cook a fillet "done" (& safe to eat), it's all personal preference of the end results.
When I pan fry "whole" fish (head off/skin on) ... I do it the same way, but add a minute of cooking time to each side, and maybe a slightly higher level of peanut oil than I use for fillets.
I normally just coat the fish/fillets with Quaker yellow cornmeal, but if I'm out of that ... I will use Weisenberger Mills cornmeal (locally ground in Midway, Ky since 1865). Been eating fish with one or the other of these brands of cornmeal since my late Grandmother fried them, back when I was a child (mid 1950's).
... cp