Levitation?

I am trying to give slow falling to a Zombie. In the past, I just used level 250; for some reason this works fine for players but not for mobs? They still move upwards no matter what level I put, as long as the level is valid (Levels above 255 or below 0 are invalid).

Currently I have:
/effect @e[type=zombie] 25 100 250

But this just makes them fly upwards extremely fast.