well i've found that live feedings really give polypterus a boost in growth.
Make sure if you use feeder fish that the fish are quarentined and gutloaded before being fed, as this makes sure you won't introduce any nasty diseases to your polyp's and it makes it so they get the best nutrition out of it.
feeding feeders and earthworms really boosted my first senegal's growth. the first month i had him i fed him pellets, frozen shrimp and fish, and he grew about 1 1/2", next month i had feeders in the tank for my nandus nandus and the senegal snagged quite a few, and he then grew 2 1/2" in less then that month, including a solid 1"+ in his best week. (you could almost tell a slight size difference every day)
but you can't ever go wrong with good food and clean water.