hi, are you a technician? just asking because science technicians can make most things work! i think you are absolutely spot on both of these pracs are ' vague ' at the very least. In the past it has been known ( not by me you understand ) to use cress individual leaves instead of duck weed to give an appropriate result after a week! but <u>really</u> if the results are a bit haywire its all down to life being life and can be explained in the students results as long as it was a fair test.Duck weed never has a problem in my pond plus Baby bio needs to be really dilute. Get some more advice on sci-tech forum, they are happy to help.