here's what I could find after I used one bucket 2 hour fresh SW combined with 2 buckets aged SW to restart a nano, this is copied from another forum:
"newly mixed SW will show slightly elevated ammonia, and is caustic until it has a chance to let the ambient CO2 do its thing on calcium, carbonates... In small amounts you are probably OK but major (25% or more) I would at least do overnite with a http://www.thereeftank.com/forums/tags.php?tag=power+head">power head</a> to vigorously mix and aerate"
So I guess it's mostly OK for regular WC but in my case it was a bit dangerous, no visible effects so far in my nano but at least now I know
thanks for all the input
misu