Ghost shrimp make great snacks for a lot of different types of fish. However, their transparent nature makes them hard to spot for a lot of their predators. I still have ghosts from about a year ago that have survived being in a 55 with my two angels (I'm assuming that they've been around for a while due to their size). I also know for a fact that my angels have eaten their fair share of shrimp. It comes with the territory. I actually view my ghosts as both functional and aesthetic. When I can find them they look really cool. They also help to clean the tank by scavenging dead or dying snails, eating algae and flotsom off of my plants and by cleaning up any excess food that my cories miss.
They also serve as snacks and as distractions from the old routine of the tank. They may not be the brightest of creatures but I feel that fish, like other captive animals in a zoo, need to excersise their natural instincts in order to be truly healthy and happy in their unnatural environment. The shrimp serve this purpose by giving my predatory angels something to hone their hunting skills on. And they do just that from time to time. It's pretty cool to watch them actually stalk and hunt the shrimp. They'll sometimes team up and actually hunt in a pair. Pretty neat.
Anyway, as for being an indicator species....eh........I guess they might die due to excessive ammonia levels or drastic pH shifts. However, being scavengers, I imagine they can survive some pretty poor water quality, like that found around rotting material under water. They do require some minerals in the water for good exoskeleton formation. They'll also benefit from hiding places. They are very vulnerable immediately after a molt since they have to wait for their skeletons to harden.
