best to get natural sea water (NSW) or premix salt water from your LFS if you need to do emergency water change. Mixing a new batch would usually take 12 hours to settle. You should purchase a good hydrometer to measure salinity. Did your ATO put too much water in or was it not working while you were away?
what!? 12 hours, not a chance. no salt takes 12 hours to mix or "settle" as you say.

