Feed them at night when the lights are off. Also, whenever I feed during the day my bichirs I walk away. Typically most of the food is gone within the hour. They tend to be slower eaters compared to my other fish.
Definitely take uneaten food out. I like to keep barbs or other fish that are large enough not to get eaten with bichirs and other messy eaters to take care of uneaten food and bits that they miss.