I think 15 W of light is too much for a walstead planted tank, and the plants are doing poorly because of the high levels of light combined with the lack of minerals in the water. And why would you leave dead plants in a tank, never mind a small bowl? No wonder the water got cloudy and yucky. I also think adding the crushed coral wasn't such a great idea since bettas like soft water. Coral will harden it. Combined with the acid from the soil may cause pH fluctuations(or cancel eachother). Those are my opinions on this.
