Drama in Shark Alley...

beginner's guess, but educated from experience in a similar scenario....

CO2 shock? - being close to going to bed, photosynthesis may have slowed,
and the introducton of new water may have forced the water into an
unbalanced equilbrium of too much CO2, or maybe not enough pushing your pH
up???

the dechlorinators are said to be safe even when overdosed to 10x the
limit... is it really?? probably varies from mfg to mfg...

Released gases from substrate? - if you vacuumed REAL good, maybe some gases
got released into the water column making it ammonia saturated and maybe
forcing some other chemical reaction?

ok, I'm out of guesses, but let us know what you find out, I"m curious...