Fire eels and the spiney eels in general are extremely difficult to pellet feed...
Advice from a seasoned fire eel keeper: start with blood worms as you are doing now. Don't worry about over feeding too much when it comes to the cubes, most of the cube you are feeding it is made up of water, and the actual substance of worm is small. If I could recommend, depending on size, I would toss in 3 or 4 cubes. Whatever he doesn't eat his tank mates will. As the eel enters the 6 inch size it is good to start feeding red wiggler worms and eventually going to night crawlers. At about a foot he should be on nightcrawlers, at this point you can introduce a pellet by deception. Go to the store and buy some frozen uncooked shrimp. Smash it into a past and grind a pellet up into it. Your fire eel will smell that seafood and grab it quickly, never knowing of the pellet it is getting. You may be able to ween into pellets gradually by reducing shrimp to pellet dust ratio over time, but a happy healthy eel has more expensive tastes in afraid.