Any sea fish is good for feeding to freshwater fish (and freshwater fish, like trout, to marines); this is to stop any potential cross infections, as it's best to feed the fish raw, rather than cooked.
You can use things like tinned tuna, but actual fish, chopped up is best. Cheap stuff from the freezer section of your supermarket is great (and lasts ages!). Or you could get trimmings/offcuts from a fishmonger (if you have one locally).