I see bad problems when ours is over 72. Some run at 75but I don't know how they do it.
68 is perfect. Too high and you will start to see algea if any light is there and pythium or root rot and there is my old friend cyano-bacteria. When conditions are right warm or cold light or dark. It's the brown snotty slime. When good and bad beneficals get out of balance you got it bad. It can become embedded in all your equipment as bio-film.

I would say look into a chiller they are awsome and you don't have to keep the whole room at 68 degrees.