When they talk about amoeba found in water that you can safely drink but shouldn't use in your nasal passages, they mean unfiltered water (untreated, not disinfected) and it would have been the first time you used it. Amoeba don't just show up if not previously there. It IS important to use filtered water which it likely was the first time or you'd be as much at risk then. Many neti pots/nasal lavage have filters to trap microorganisms. I understand the fear of using older water but would not be overly concerned. The bigger question is if it was disinfected water to begin with.