We have tried at least 10 water fountains because Miro is a picky water-drinker.
Some cats might prefer drinking from human's cups. You should check if they are "cup-drinker" or "fountain-drinker" before you buy things.
We use Brita to filter the water and put the water directly into the fountain without further filters (You can buy the commercial filters if you want to)
You need to change the water at least once a day. Wash the water fountain at least once a week (in dishwasher or hand-wash), remember to clean the pump because sometimes hair/random things get stuck inside and stop working.
Here are our favorites: