Thats a lot of ammonia...From just fishless cycling my 55 I would guess I would have to use 1/4 cup pure ammonia to hit 8ppm. Unless you werent checking it for a long time I dont think over feeding could cause you to go from 0 to 8ppm. Even a large dead fish in there shouldnt cause it to go that high. I guess I would buy some cheap fish and a bottle of cycle and see what happens. May as well do a good vacuum of the gravel since It doesnt appear there is any BB there anyhow in case it is rotting food causing it.