If you're looking for an answer specific to your team, scoring system and lineup requirements and league size would be helpful. Also, those team-specific advice requests should be posted in the Assistant Coach forum pinned above.
However, I can't see a situation in which Tank Johnson would have any value. He's a nose tackle in a 2-gap 3-4 front, and a part-time one at that. If you're in a 16+ team league that requires at least one DT in the starting lineup, he might have spot starting value. Realistically, though, he's probably a 25-30 solo, 0-1 sack type of player.