I got 2 gallons of drinking water to start. I have a brita pitcher, so I'll run my tap water through that when I do the changes. I wanted the gallon jugs so that I can fill them and let them sit to get to room temperature when I'm ready to change the water.
Remember, too, that it is easier on the fish if you change the water about half at a time. Especially if there is much difference in the water temperature.
I had mine on a table but the lamp light gave just enough heat that he didn't do well, and positively got nervous when he knew the tank was going to be changed.
It took me a couple of weeks to figure out that that was NOT a good place for his tank!