I am not an expert, but only feeding the fish once every two weeks can lead to over eating. Polys are already gougers as its is. I try to feed my two Sens only sinking carnivore pellets or red wigglers. I have noticed that my Sens will "blow up" and look that way for a couple of days until the food is digested and they poop. It last for days when they get a hold of some of the cichlid pellets that are supposed to be eaten by their tank mates. I feed each one two carnivore pellets or some chopped red wiggler daily. The only other food I have used with similar results to what you have pictured and described your bichir as having were freeze dried river shimp. The shrimp seems to take about 4 days to pass and my sen had a huge lump in his stomach just like yours. Fast the fish for two weeks like you are doing and then try some Hikari carnivore or massivore pellets and see if they work.
