i think you guys are failing to understand that once one rats figures it out...the others will learn very quickly.
it's the same concept as putting a new water drinking valve in the tub....none will mess with it, until one gets curious enough to bite on it....when it see's water comes out it will continue to bite to drink....the other rats see this and learn to drink they must bite the valve......cull the entire tub, replace with a new one and go from there.
Pitoon