Temperature difference in deep reef tank

I am about to set up a 90 gallon 36*24*24 reef tank with lps and soft corals. The plan is to run a 24inch long ballast of T5s. I was wondering of anyone with a deeper tank has experienced cooler waters towards the bottom or if it all stays pretty much the same? For example, the top is 82 and the bottom maybe 78, or is it not even a temperature fluctuation of 4 degrees?