How long does it take for the chlorine in the tank water to go away? I recall some stuff I used to add to new tanks, counteracted remaining chlorine and also de-stressed the fish, sorry but don't remember brand names but generically it is called "de-chlorinator". A 10" Plecostomus (sp) is pretty big, I assume that's what it is, would be a shame to lose him.
http://www.freshmarine.com/setup-freshwater-tank.html