first discard any old filter media , yes u will have to start over again.
soak any filter canisters, hoses, pipes, heaters, thermometers, nets in a solution of bleach:water > 1:20
run filter for an hour in this solution to make sure anything inside the filterhead is dissolved.
empty tank let it dry out 48 hours then wipe interior with Isopropyl alcohol..
also i suggest you dont feed live fish anymore most LFS are full of worms and parasites.
quarantine any fish with metro and prazi before adding them to your tank