If you're loosing water to evaporation, be careful topping it off. When water evaporates, the other stuff in it does not, so you end up with higher and higher concentrations of amonia, nitrates, bacteria, and other bad stuff.
Of course, everyone has to top off a pond from time to time. But make sure that, once in a while, you also remove some of the water before you top off, to get rid of accumulated toxins.
If Joyce pops in here, she know a lot more about this subject. There has been a thread about it some time ago.