Peas have been used for years to help Betta and Goldfish, especially, with constipation. While it's true Daphnia might be a better choice peas are fine if only a tiny bit is given. I used them for years when fillers were the main ingredients in Betta food and never killed or harmed a Betta. Way back then they were part of a weekly diet because we knew the fillers had to move on out of the Betta's system. Such as frozen foods were not readily available and were so expensive as to be prohibitive.
Flaring also helps get a Betta's gut moving. Frozen Daphnia you can find at many pet stores.