You should be using 1 tank for both normal and heroic modes unless you try that weird strategy where you use two tanks and two healers and save Nozdormu for last. 1 tanking normal mode should be a laughable encounter, especially with a Death Knight.
To effectively tank as a DK on Madness, this is all you have to worry about:
Corruption: Pick up swiftly, pop Bone Shield, and use Outbreak. Try not to burn all of your runes until Impale begins its cast. When Impale begins its cast, spam Death Strike as fast as you can. If you get 2 or even 3 in, Impale will not even tickle your health bar. Blood Shield is that awesome. When tanking the Hemorrage adds, pop AMS as soon as you begin tanking them. Just lay down Death and Decay and Blood Boil them. Rinse and repeat for all 4 platforms.
Elementium Terrors: Drop DnD where they spawn, pull them into Nozdormu's Time Zone, put diseases up, and use Death Strike as much as possible. When Tetanus hits a 5 stack, use AMS to drop the stacks. This should allow your DPS to have killed one of the Terrors. DPS should kill the other, you should use Dream, and bam....you've won.
For Heroic mode, nothing changes except for the 2nd Impale on each platform...you musn't eat it. You should have a Rogue, Hunter, or Shadow Priest 'tank' the Impale by allowing them to be the only melee target right before Impale goes off. Rogues Vanish, Hunters Deterrence, and Shadow Priests Disperse. Once that has happened, pick the Corruption back up and finish it off. Death Knights are by far the best tank for this encounter as well as darn near every other encounter in Dragon Soul.