500 gallons is a big aquarium, very cool. If you are overwintering RES get a heater and keep the temps above at least 60 degrees. RES are very susceptible to fin rot when water temps drop below 55 degrees.
I think that's true with the whole centrachid family although perhaps moreso with redears. Many fish producers will not seine them in cold water.
From my limited experience stress + coldwater = fungal issues. Stress + warmwater = bacterial issues.
