EPTDrop units on top of the tanks and the most effective unit for that is the baneling, or borrowed roaches popping up on top of tanks or the units protecting them, or hell both. They can't kill what they can't see.
If you like the heads on approach then make 400 zerglings and i'm sure that will cause the tanks to kill each other as well as any units around it. If they have hellions tho you lose.
Last and most effective option is dropping their main, natural, or just an expansion in general. Tanks can't move or they become very ineffective. Also Zerg players often have a excess of minerals so make extra overlords in case you lose a few. That is how you beat tanks. By simply avoiding them.
