Help another temp question

[DEAD_BY_DAWN]

well so far ive been able to keep a handle on the temp,this was until yesterday when it hit 84 nothing seems bothered by this spike but at the same time is a cause for consern for me ive placed a large floor standing fan so it blows onto the the tank,im not sure how much it will help,the temp this morning was a nice 78 but a 6 degree difference during the day,will this be fine ? ,im sure coral reefs have the same thing happen during summer and winter months,but can i do anything more than that ,btw my tank is covered with a glass sheet and im unable to remove this due to my lighting arrangment.