As a beginner fish owner myself, I definitely agree with you; owning them is far more difficult than pet stores make it out to be. Thor is definitely the fish expert around here, so can probably help you out better than me, but based on what I know from research I've done, I'd say to be careful about using bottled water. I'm sure some bottled waters might be alright, but at least the bottled water where I live is not treated properly for anything but chlorine and is basically glorified tap water, and I think could end up being dangerous to use without treatment. I've read an article specifically on using bottled water here before which seems to suggest against bottled water as well.
For treating water, I use a water conditioner combined in regular tap water. The conditioner is very affordable at pet stores, and removes both chlorine and chloramine. Definitely a less expensive option than using bottled water, too. The one I use is Nutrafin. The pet store told me it works instantly, but I usually leave it in the water ready to change out for a few hours just to be safe anyway.
Now, I don't own Siamese fighting fish so I don't know their water needs compared to my goldfish, but water conditioner for fish is so inexpensive (at least from what I've seen) I think that using that is the best bet really.
Also, when getting your tank, I'm not sure if this is standard or not but it was recommended to me that I let the filter in the tank run with the water for a few days as preparation for the fish. The store here said that adding a fish to a new tank can lead to complications because the fish isn't used to the new environment, and "new tank syndrome" is an occurrence a lot of fish owners don't know about. I am not sure how accurate that info is, but best to be safe then sorry I'd say!
