It's been chilly down here in South Florida and my tilapia aren't at all interested in food. I'm heating the tank but the temps in the tank are only in the high 60s. I have to run my system intermittently so the ammonia levels don't climb but am leaving it off during the night when it's the coolest.
Should I be concerned or will they eventually eat when they really need to?
Well, when it's cold, a fish's metabolism slows way down, which means they won't eat as much. If they're unhappy or stressed, they don't eat either. If everything looks good as far as water conditions go, then it's probably just the cold.
Tilapia can go for a couple weeks without eating, if I recall correctly.