Flood tank form

The Flood Tank Form is a type of Flood from Halo 3. It is a Pure Form, originally derived by mutating from the Stalker Form.

The Tank Form is similar in appearance to the Drinol (possibly inspired by the Drinol, who has never made it into any of the games), or a Hunter. Tank Forms are slow in comparison to the other Combat Forms but they make up for it with relative resilience and their devastating melee attack as they cannot pick up or use weapons with their arms that are shaped into hardened spikes (as opposed to whip-like tentacles exhibited by other Combat Forms). On occasion these large combat forms eject Infection Forms from their 'mouth' as if regurgitating them (not explosively like the Flood Carrier Forms and possibly only on harder difficulties) as a form of short ranged attack, or an attack to counter act bubbleshields. Like all of the Pure Forms, The Tank forms cannot be dismembered, only disabled.

Killing Tank Forms
The best way to take this flood form down is by shooting in the head/mouth which it often covers with one arm for protection. If you hit there with a projectile weapon (ex. Battle Rifle) it will count the shot as a head shot. All flood Combat forms have this supposed "head mouth".

They are extremely tough, and attacks that destroy normal Combat forms with one hit deal far less damage to a tank form. They are able to take multiple hits from the Gravity Hammer and the Shotgun, though the Energy Sword seems to make shorter work of them, as will a melee attack in the mouth, mostly killing them in one hit. Since they are very slow and unable to rush the player, the Flamethrower or Firebombs are a simple, ranged, one-hit option attack against the Tank Forms. Brute-shots are also extremely effective against them because the melee attack with the Brute-shot is an instant kill against them on all difficulties except Legendary, which takes one shot with one hit. Plasma grenades are also very effective only if they are placed near the head.

There is a more unconventional method of jumping onto one's head, and either meleeing them or using a powerful weapon, as they are unable to attack above them. Note that this only really works if the tank is alone, thus nullifying any risk of damage from other Flood forms.

Trivia
Flood tank forms bear a strong resemblance to the comic book character, Man-Thing
 * Juggernaut, Flood Combat and Tank Forms have a "mouth like" head, where the sensory tentacles are located.