fish dying after installing sump

Question
ok here goes, I've had my 120 gallon saltwater tank up for about 2 years and never a problem, so i decided i wanted to put a sump/refugium on it,so i could get rid of my fluval fx5, so i went with a 55 gallon tank for my sump.about three days after getting it all up and running everything just started dying, in 4 days i've lost 4 tangs, mated pair of clowns, two wrasses, foxface, porcupine puffer, and a whole wack of corals all that is left is a blue damsel and seems happy as can be, swims around takes food well, very healthy.up until now i'd never lost anything in my tank all my levels are bang on. i just can't figure out what went wrong. i did move some rock down into my fuge which i think may have disturbed the balance and after research i've read that the tank may be going through a recycling stage but i dunno. i ran the set up with fresh water before i put it in. any help would be greatly appreciated.
thanks
brad,
I'm a canadian boy too lol

Answer
Hey Brad

Did you totally remove the FX5? im thinking that might have taken away alot off biological filtration and caused a mini cycle, therefore killing alot of livestock. it would have been a good idea to run the sump and FX5 together for a few weeks until the sump got more mature. if you still can, stick some of your FX5's media in the sump

good luck!