Is it too late to save my goldfish?

I started water changes two days ago, and after 4 changes, the fish seems much more lively. He ate some food but spit the pellets (designed for goldfish) back out again. My "bowl" is actually 4 gallons. If I got a filter within 72 hours, will it still work?
Also, I have a dechlorinator (it says it changes it into ammonuium), and I was wondering which type of water I should use. I have basic tap water, and I have this "filter". Its this "filter" (not aquarium filter) that my family uses to filter tap water for drinking. Which one would be better? I think that the "filter" adds some minerals.
With the filter, how often do I have to change the water?
Many thanks to everyone for the help!