In my experience with betta's, the guy at the fish store mentioned that betta fish can live without food for a week or 2 but they need to be fed a balanced diet daily. There are betta food pellets specifically for betta's to keep them healthy and maintain their luxurious colors and frilly fins.
The original question about the water application reminded me of the fish combined with the plant roots of Spathiphyllim in water only, which worked great for the fish / vase application. I added a layer of blue glass rocks at the bottom of the tank in a 24" h x 8"w glass column vase and a light , it was also a living lamp.

It was designed for a local flower show and lasted several years.