From what I've heard (could be a myth... who knows) otos are a special case because to harvest them people pour some kind of neurotoxin into the river upstream to temporarily paralyze them. It makes it much easier to catch large numbers of them. So even though the ones that make it have survived the process, they have some degree of damage to their systems that acts as a ticking timebomb.
I certainly wouldn't put it past people to do such a thing. There may be other species who get the same treatment.
Otos only eat so many types of algae (if it grows on glass, it's probably the kind they'll eat, not counting cyanobacteria which isn't an algae anyway), but what they do eat they eliminate fast. For algae control of that type, they're one of the best species you can get.