Plumbing - Particles / Debris In Faucets and Toilets - Water Softener Going Bad?
No disrespect to you or your plumber, but I would be suspicious of any person who is able to predict the demise of an appliance by reading mineral andiron deposits in an aerator screen. It is common after replacing a water heating tank (or doing any work on the plumbing) for debris such as hard water minerals, silt and bits of metal and rust to break free and "go with the flow" (as it were). Their final resting place is the faucets aerator screen. Of course, my assessment is based on what information you posted. If you could include the model and how long it has been in use I could offer a more insightful answer.