Prime should pay us for recommending them, haha! I have never seen anyone here recommend anything but Prime.
It's 2 drops per gallon. It is the cheapest for how long it lasts. The smallest bottle has a dropper lid. I have a bigger bottle, and I just use an eye dropper. It should last quite a while, even if you get the smaller bottle.
I also treat the water before I add it. I, personally, don't like the idea of untreated water in my tank for any amount of time. I treat it, swirl it around with my hand, then add it to the tank. No worries that way.