what are your water params? a fish died and its not as easy as just replacing it. practicing using a QT before introducing into a display tank is also a very important step. i personally wouldnt rush out to buy a new fish anytime soon unless you want to risk it dying too. i would observe the clown over a short period and see how it does. i also wouldnt feed flake food, but thats my choice. a good mix of different quality foods is best for health and their ability to fight off sickness. alot of foods are "dirty" with un-wanted phosphates, nitrates, minerals... and should be pre-rinsed prior adding to the tank. this will help prevent algae issues later down the road. over feeding should also be avoided, and this will help keep those bristle worms your talking about within a reasonable number. if they arnt able to get big or in large numbers ( which is a dead give away of over feeding ) they become a good natural part of the clean up crew.