Depends on the fish-BUT it's a lot of work to properly take care of it no matter what fish.
1 betta - change water daily if you overfeed (I have two set up next to each other and move my betta every day)
3 whiteclouds, guppies, or endlers. every other day 50% water changes and need a sponge filter. Will desperately need a sponge filter. Chances are you will get babies unless you get all males-the babies will need to be moved.
NO GOLDFISH-EVER. Goldfish grow to insane sizes and need at least a 30 gallon tank to be healthy and happy. Please don't do it.