Did you ever rule out a sulfide pocket? If the dsb
isnt aerated properly, "lumping" will occur and trap pockets of sulfides.
Excerpt from Dr. Rons article on DSB
Lump formation is a disaster for the biological filter. The lumps restrict water flow and trap organic material where it can rot. Additionally, lump formation shuts down the biological filter by covering the bacteria and preventing them from metabolizing nutrients. This, in turn, causes the tank nutrient levels to skyrocket.