Aging is variable - if you aerate in a shallow bowl, 1/2 hour may be enough. I you age in a higball glass, more than 24 hours would be more likely. Aging in a capped bottle does not work - air exchange is critical, as is exposed surface area versus total volume.
I've never had an established tank with a true zero nitrate reading (<1ppm) unless it was heavy planted and barely stocked. My tanks read <10ppm regularly as they are heavily planted.
No tank with low nitrate is likely to be in OTS. The combo of insufficient water partials to clear nitrate and at the same time restore the KH/pH to near-source water levels pretty much defines OTS. The TDS is usually way up also (from top-off to replace evaporated water without sufficient changes to dilute the increased density out). The high-nitrate, low pH, high TDS water is endurable by the old fish as it happens slowly and they adapt. That same water is too alien to new fish, they go into osmotic shock and die. Most folks do not have TDS test equipment, but nitrate much higher than source water, pH & KH much lower than source water is enough to diagnose OTS as a cause for new fish deaths.
You do not apear to have detectable nitrate, so OTS would not be likely.
