Your water pump is leaking, I have 191K miles and mine was always needing fluid in the reservoir. It was when I blew the small hose coming out of the radiator back to the engine, that I found the telltale signs of a leak coming from the water pump. The water pump has a little hole in it that when the water pressure gets to high it lets out water there, well as you can guess after time it starts letting water out all the time. Another thing to look at that happened to me is the little pipe that connects your two upper hoses has corroded and is leaking, that happened to me also and you can only get that part from Nissan. I paid alittle over $10 for mine.