I'm with the group that thinks hemp is effective enough on its own, I do alter it in a fashion when I use it for carp, what I do is to keep it in the cooking water then leave it in the sun for up to a week. It sort of ferments similar to how a yeast works, getting a frothy covering telling me it's ready. It proper stinks that could be the attraction.
Carp, where I fish love what I call cheesy hemp, fish in the margin with a corn or meat hook bait.
I can't see how tench wouldn't take it if you dare try a little to see if it would work ?