how often should i change my water for my water cooling?

depends on the manufacturer's reccommendations. i believe every 6 months or so you have to perform simple maintenance involving shutting the system down and repressurizing it. what water cooling system do you have?

i have the zalman reserator

whats a water cooling system and does every pc have one?

Are you serious?

It's a system for using water instead of air to cool cpu (, gpu, hdd and other parts that create heat). It is not standard on regular pc's, nor do I believe it'll be for some time to come. The cost is too high, and the risks involved will deter your average joe for the time being...

I would recommend a tiny bit of antifreeze in the water and using a surfactant booster (they sell in the antifreeze sections of auto parts stores). Not only will you increase cooling capacity, but you'll kill microbes with ethlyne glycol.