Ok so I had an idea on getting the stuff out of the tank. Only about the bottom 3 inches has the old black stuff. I went to the pet shop, got some 1/4 inch rough gravel. Filled the tank about an inch deep. There is room around the baffles to get it evenly distributed. I tied the two ends to a chain and hung it from a tree. I then went back and forth, side to side. I did this off and on for around an hour. Flushed it out, gravel all came out rather easily if you angle the tank right. My result after an hour is crazy good. I am going to do another round tomorrow and I think it will be good to go.