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That usually shows it needs a good flushing. This can happen over time depending on the mineral in the water where you are. You can try taking out the drain plug (after you turn it off) and flushing it with fresh water for a time and get all the lime deposits out. If this persists, you can use about half and half white vinegar and water and leave it sit for a time. Best to do this when you aren't using it. If you need more please get back to me and include the make and model of your water heater. All follow ups are included in our fee.
Hopefully, you got this resolved as I didn't hear back from you. You can always return to this posting again if you Bookmark it and we can do a follow up. All part of our service and please take time to enter a rating as that is the only way I get credit for my time spent helping you. Thanks! Be safe........