You can do that but it's rather bad for the fish as untill you clean them they are swimming around in rising levels of amonia, Which is deadly once it reaches a certain level. With a little filter you wouldn't have to clean it as much. Tap water is bad for them too unless you use some kind of water treatment.
All depends the amount of effort you want to put in really. Remember fish are living creatures same as a hamster, rabbit, rat (although considerably less gnawing is involved lol)