I had success feeding 3 times a week. I could tell when they were hungry because they would start leaving their caves and hunting all over the tank. After they were fed they would stay in their caves and just watch the other fish for a few days. I agree with fijsrfr on types of foods. I was also taught that if feeding silversides, stuff a few quality food pellets down their throats before feeding to the eels. You can also pre-soak the pellets in vitamins if your like me and don't have a life
Good luck