We learn in that water collected in a vessel is considered Mayim Sheuvin (which is invalid for a Mikva) provided that he had the intention of collecting this water in the keli. Why is intent necessary here, if the water is in a keli why is it not automatically considered Mayim Sheuvin?
the mefarshim in mikvaos discuss this at length. to leave a brief reply; the ra”sh learns that if the water would go directly from the vessel into the mikva it would make it pasul regardless of the intent of collection; however there is a way to ‘fix’ the water by letting it run on the ground. if the water has been collected with intent it cannot be ‘fixed’ in this manner.