sounds like the heat exchanger is breaking up due to poor pool water chemistry.time to get a new heater
the black stuff is metal

Domingo is correct.
I'd feel certain it's chunks of cast iron from the manifold ends of the heat exchanger due to aggressive water chemistry.
You can replace the heat exchanger if you choose to or buy a new heater.

See ya,
Kelly

Don't put good money into that old heater. It's like putting a new transmission a 1990 Chevy Chevette with 175000 miles on it.

Scott

Yes I had a major phosphate problem. Should I get another Minimax 400? Best price I found was just under $2,500. Or should I get a different heater?

NOOOOOOOOO! It's been discontinued for a few years now.

RayPak/Rheem heaters are far more reliable and proven oven the years.

Scott

Which RayPak/Rheem would be the equivalent to the Minimax 400?

Here's the RayPak. The Rheem is the same except of the way vent stacks are attached and the distribution channel.

http://www.raypak.com/Raypak_Digital.htm

The 400K with the Cupro Nickle heat exchanger and polymer headers is what I think you want.

Scott