[ThRoewer]

I have my main system on a seasonal cycle with 24 the lowest in the winter and 27 the highest in the summer. This is for once to simulate the natural seasonal changes, but also to save some energy.

As for what extreme temperatures fish and inverts can handle - I've seen shallow tide pools on the North Sea cost that were blasted by the sun and outright hot (30 °C wouldn't be surprising), yet the fish and shrimp in there didn't show the slightest inconvenience. Mind that these were temperate-cold species.
I think the temperature alone is not always the issue but rather the oxygen depletion it may cause. Though if there is enough surface area to ensure a sufficient gas exchange, the animals may very well hande temperatures well above their comfort level.