My Gars get these on their eyes every now and again and like you have dicovered a large water change clears it right up. I have tried to let the WC slide a few times to keep with a maintanace schedule and the condition would either stay the same or get worse. 30-40% WC and the next day most of the dicoloration is gone which leads me to believe it is more of a water quality issue than an injury.
Like you I have a large bio-load due to some large fish in one contained system and even with weekly maintanace things can go south in a few hours requiring an extra WC.
My feedings are done on a schedule and all food is measured each time, no idea why this happens when everything is run like clock work, just glad it is an easy fix when it does.
Side note: all times this has happened to me I never removed the fish with the eye discoloration from the tank, never raised the tank temp or ever added meds, just a simple water change and all was back to normal in a day or two.