Pro forma, make sure it’s the water heater causing the odor and not something else. There is a simple troubleshooting formula on my smelly water page that will help you with that.
If you’re satisfied it’s the heater, tell me what brand you bought and I I’ll tell you which anode to get.
The particles could be left over. They could also be from galvanized piping, if you have any of that. A third possibility: if you have braided stainless steel flex lines, it could be disintegrating rubber lining.