We are having it checked to see if there is any microbial growth in it. For you non-biology people, it really doesn't take much for algae to get going. So far, we have seen NO growth in our samples. Some of which have been sitting on the shelf for months. They are sealed though. We are going to leave some in an open container to see if that has any effect. Outside of that, there is nothing in the nanofluid to break down. The only thing to worry about would be evaporation, but that would only be the liquid. So, you can just top it off with DI water. The nanoparticles can't escape due to evaporation. I have had nanofluid in my system since June with no problems. I don't do any serious overclocking, but it has been stable. I have NOT turned my system off though. Even when I go out of town for a week I leave the system running.
If we do have microbial growth we are going to have an additive added that will prevent that.