Two years ago I had around a thousand cherries and half a dozen mystery snails in a 75 gal. that heavy planted when brought in some of my parrots feather from the pond for winter and ended up with leeches. Needless to say, soon after the leeches appeared my baby shrimp disappeared and my mystery snails started dying off. I just moved a few hundred cherries or so to my 125 gal. and moved in some of young gold fish that I had to bring out of the pond for the winter. It took a while, but they took care of leeches, but also took care of the most of cherries and plants as well. Moral of the story, I will never bring pond plants into my aquariums again! Do you know how you got your leeches? I hope your colony of cherries makes a comeback as good as mine have.