Sediment in Water Heater

Hi,
Rheem Performance Gas Water Heater ( not even 2 years old ).

My problem is I have sediment in the water heater tank and cannot get rid of it. I flushed about 2 gallons from the heater tank into a bucket,poured it into a white sink and on the bottom of the sink it's a light brownish sediment that feels like fine table salt.
I have drained the tank,flushed it and still the same,,,power flushed it, back flushed it. I have done this about a dozen times now and it's still there.
Well water and softener,with a all house filter,
The filter turns black in about 6 -8 weeks. I know that's Magnesium,but I don't think that's the problem. My cold water is very clean ( no sediment ) the toilet tanks are clean ( no sediment ). It's got me,where is this crap comming from.

I recently pulled both my elements out and cleaned them (ended up replacing the lower one), replaced the anode rod (that was reduced to the size of a few pencil leads) and flushed it out a few times. The other day, when replacing the drain with a brass one, I noticed a layer of sediment (and whatever else) at the bottom of the tank. While flushing it with the drain off, I vigorously shoved a long dryer brush in the hole, finally cleaning out all the crap.

Even though it's only a couple years old, it doesn't take long for water heaters to get messy. It may take a bit of time to get it clean (if that's the only problem). There are plenty of videos out there that show how to clean water heaters. A lot of people use vinegar. Good luck.