Almost all tap water has some level of ammonia, and of course the fish waste creates ammonia. That's why it's important to change the water often, especially if they are in small containers. The ammonia levels can build up and become toxic or cause illness very quickly in small containers. There are ammonia test kits (and nitrate/nitrite etc etc). But again that stuff is not cheap. Fishkeeping can actually be a very expensive hobby if making sure they will have good conditions and live the longest life possible. Unfortunately a lot of people get fish thinking they are "cheap" or "easy" pets. But if a person wants their fish to live longer than a few months, more investment and effort needs to be put into it. Betta fortunately are still easier than most fish since they can tolerate higher ammonia and nitrite levels than many other fish, but they still often get fin rot and other diseases if not in good conditions.