It depends on the type of sunfish and where theyre from. Their is a subgenus of Longear from Louisiana and Mississipi that does well in tanks that are in the high 70s- low 80s (Ive read) then there are Northern Longear which would do much better at lower temps like the upper 60s to mid 70s (Ive heard). If you have a Northern Longear and live in Southern Arizona and dont have your airconditioner always running 9 months out of the year you would need a chiller, if you live in Minnesota and run your airconditioner in the summer you will probably be fine (if your tanks inside). Redear, Fliers, Bantam,Spotted,Florida Bluegill, Louisiana Bayou Longear and Dollar Sunfish do well in warm tanks, whereas Pumpkinseed, some strains of Bluegill and Longear do better in cool water, and it all depends on where the Sunfish is from if its a Orangespotted, Redbreast, Warmouth, Green or Mud Sunfish. Since Im from Mo. and had Green Sunfish they were pretty resilient with the warm or cold and I kept them at room temp. (80s in the summer, upper 60s in the winter) and they did fine.