I would say it's tough due to the variable of putting them into a previously used tank with established media. You could potentially have a cross contamination issue. I don't know that the slime from the canister would have caused this. Believe me when I started with my fish I opened a slimy filter many times and never had an illness. You are doing right by the large wcs daily
I don't ever feel comfortable suggesting meds so hopefully a bump will get you some response