Okay, I've heard many people say that there are constant water changes in the ocean. WHERE DID YOU GET THAT IDEA!!??? The water that replaces the previous water is just as used. It's like the current from a powerhead. The fact that new water is coming to the area has nothing to do with the freshness or cleanliness of the water. If anything, it proves that you don't ever have to do a water change. You will still have to add supplements though. The aquarium doesn't replicate the decay process which puts nutrients back into the water.
About the issue of temperature...humans can live in a wide range of temperatures. That doesn't mean that it's good for us to experience a wide fluctuation daily. Many virii are contracted when a body has to change tolerances thus the saying "you caught a cold". Corals, like people and other animals, may be able to handle a wide range of temperatures, but they have preferences that don't favor daily fluctuation. If you keep the temperature more stable and have a seasonal temperature change your corals will be happier. Millions of years of evolution have created these animals. In order for them to adapt to temperature variences they only need to experience it once in a while (seasonally).