Hi Denise
I feed raw sardines, ungutted with the heads on!
In the book he says if you feed a lot make sure it's the whole fish - eyes and entrails as well and supplement with Vitamin E. It is recommended not to feed a lot but make it part of a mixed diet. He particularly mentions sardines but mackerel is an oily fish too so you are providing some of the omega 3 essential fatty acids.
Enjoy! - don't forget the head next time