We generally use the vegetable material as a laxative not as a meal and it works. I have seen many bettas saved by this method. I recommend it but not as a meal replacement unless they are not eating anyway and are in severe distress and then I have been known to recommend it be given for a period of a couple days to clean them out. Overfeeding may constipate them and 76 degrees is a bit cool for their metabolism. The digestion will be a bit slower and constipation could have been the cause. I am so glad it was just the little pout for the want of the favorite plant. They do get very attached to their favorite items.
Wild bettas may not be so finicky but pet bettas do get very spoiled and decide very quickly that they like what they like and want what they want when they want it. They also have a tendency to become depressed if not humored. People call me silly but I am with my fish 24/7 and can watch their behavior and notions very closely. It is obvious if you spend time with them and make over them that they have definite personalities and they know what they want.