I'm pretty sure you can add it right after you add the water to the tank if necessary, but you might wait for another opinion since me being 'pretty sure' isn't worth the life of your fish.
I've mistakenly added untreated water to my tank and added conditioner immediately after with no ill effects.
Some members use a Python for water changes, and I know people have mentioned there's a way to add the Prime.
At any rate, this'll get bumped up so someone else will see it and hopefully answer you in a more confident way.
I'd love to see some pics! 150g wow!
