When in Arx, there is a barrel at the lowest level of the blackhouse where if you try to move it, it'll say "its stuck to the platform". Note a symbol on it. Now head outside, past the graveyard adjacent to the house and down to the seaside. You'll find another barrel with a similar symbol. Put some fish in that one, and cast the scroll on it. It'll be sent to the barrel in the lowest level of the house. I've tried the reverse way (basement to seaside) but it didnt work. Also, tried with different types of fish, including voidtainted fish, nothing significant.
You literally just sent the doctor his lunch.
I only found this out after killing the doctor, so I'm not entirely sure if you get anything after doing that while he is alive. But i highly doubt you'll get anything, because trying to get to the lowest level of the house will aggro everyone in the house and you'll be forced to kill him either way.