My theory behind adding prime at the end is... well we know it takes seconds/minutes for prime to work, to even dechlor a whole tank, BUT does anybody know how long prime works for before it exhausts itself and stops working all together? I would hate to think that if I added it at the start of refilling that it could stop working or be less effective before the tank has been fully refilled. Thats why I add it at the end. Its just a theory mind and I aint had any problems yet. XD