I need a bit of clarification in terms of dosing my tank with water conditioner. I had a 120ml (4oz) Nutrafin water conditioner and it ran out after 4 water changes.

I have a 15G tank and I'm wondering if I'm doing this right. When I do water changes, I put conditioner in the new water which is in the bucket and then pour that into the tank. I do this with each bucket full of water till my tank is full. Is this right? Or am I suppose to put the new water in the tank first and then give the required amount of conditioner to the entire 15G tank?

This is also where Seachem Prime comes in. I am planning on buying Prime but I'm wondering how long would it last me? More specifically, how much Prime treats 15G of water?