First thing is to check the date on your water heater. It is often the first part of the serial. If it is a letter code, tell us the brand , or go the the website of that manufacturer to decode.
Anyway, if your water heater dates to the early/mid '90s, you are suffering the infamous dip tube problem. Google dip tube to find out all about it.
To confirm did tube pieces, place some chunks in a teaspoon of white vinegar. The plastic will not dissolve. If your problem is actually mineral deposites, they will dissolve.