Tips from a tank to tanks on Fester:
Change taunt after you get 9 stacks.
Most important: use your cooldowns 1-2 seconds before 3rd Inhale Blight comes up.
Fester gets tons of haste after each Inhale Blight and there are 3 inhales before it resets and start again.
So, remember to burst ur tank cooldowns on 3rd inhale, otherwise your tank will be killed.
Edit: also remember that after changing taunt, the first tank should change stance to battle/presence to blood/ removing rf / changing to cat form...to generate less agro. This is due a huge buff to the tank damage and when your OT taunts fester, you should do less agro or MT could get boss from agro, get the 10th stack, die, explode and wipe your raid.
Edit2: but, with 6 dps your raid should be doing with easy this fight...seems that your dps team is the key and they should try to improve their dmg or just replace some.