Please don't keep your betta in a bowl. They do best in tanks of 3 gallons or more. I agree that a marimo ball is not a suitable replacement for filtration, plus you wouldn't have the aeration to cycle the tank. Get a decent sized tank, heated, filtered, you can still have a marimo ball if you want, and cycle it before getting the betta. A tiny, uncycled tank and lack of filtration is asking for disaster.