I do not recommend feeding them solely pellets, but what I do for extra nutrition is, i take a anchovy or a sardine and fill their bellies up with vitamin soaked pellets and then feed the anchovy or sardine whole to the eel, I've been doing this for about 2 years and have seen quite the growth and colours in my eels.
Just for added information, i make the pellets a paste, make sure to choose a brand which is high in fats and protein aswell as ingredients listed as whole fish instead of just a carcass (fish meal)
I recommend doing this every other week, but also change the type of foods you are feeding such as pieces of mackerel, molusks, whole shell and head shrimp, squid etc