you really shouldn't ever change a LOT of water all at once, it throws your fish off. so if you want you can move your pleco over to a smaller tank but i'd put him in with mostly water from his old tank, and then just change it in small amounts over the next few days. keep a close eye on him. how much water did you change yesterday? and, not to ask an idiot question, but did you make sure to use something on your water so that it is dechlor-ed, etc?