Now I see what you're trying to do with the Leech! Leap then hold still digesting.
Which means that the leech isn't a skirmisher, it's a lurker. Pounce then very little as a standard pattern. But as written hydrostatic mitosis simply isn't going to trigger; no one uses save end effects on minions.
Which raises the damage it does on the leap to 8 (making one scary minion on the alpha strike).
Hydrostatic Mitosis: Free Action.
Trigger: The Leech would be reduced to 0hp by an enemy in its aura.
Effect. The Leech makes a saving throw. If it fails it dies. If it succeeds it splits into two Leeches, both on 1hp.
No need for extra damage; that's a big and fluffy enough effect to be memorable. And don't make a silt-farm to give extra XP; the chance of splitting is included in the XP budget anyway. Also no need for the clunky "Special" line - making the trigger a free action means that it triggers even on the leech's turn.
And if you want to get really evil with the leeches, add the following line to a Silt:
Avatar of Silt Aura 10: All Silt creatures in this aura increase their dehydrating body to Aura 2.