Yes, the filter is in the resevoir. It doesn't take much, as you can really only change what is in the resevoir itself. To get the old fluid out, use a turkey baster or a syringe that you can get for free at the pharmacy counter at the grocery store. Tell them it is for measuring your kid's medicine. If you are real anal, you can suck all the old fluid out, fill it, and repeat the process a few times after running it for a while to change out a larger percentage of the fluid.