I have one of those little ecosphere globe things. Its the size of a large orange. It has a twig, some algae and a little ghost shrimp or similar. My wife got it free at a convention 12 years ago. The temp of the water has been as high as 85 and as low as probably 50. When new, the globe was 100% full of water. Now, its about 75%. There is some small evaporation out of the rubber plug.
The little shrimp is still swimming in circles, all day, every day. The life expectancy on the package said 2 years. 12 years and going strong with nothing added.
As for my reef, I've gone a week, with no ill effects. I feel bad for the fish though.