canister filter mistake are not?

[christog30]

I bought a new fluval 405 canister filter yesterday,so I took my magnum 350 off and put it away.Last night about 2 hours installing my new canister my fish start breathing hard and a couple die.I saved a few by putting them in my 29 gallon seahorse tank (doing fine now).My question is do you have to let the old canister stay on until the new one is broken in? I need to do a water change now my levels went up after last night (i think a mini cycle). So any help is appreciated.

 

[christog30]

Any help? I want to put my fish back in here but I want to be sure first.If I did introduce a contaminant will a uv light kill it? If not how big of a water change is suggested?