As someone mentioned testing the salinty in the bag is also important. The LFS may be keeping the tanks salinty a lot different from yours... Also you need some test kits, for the price of those three shrimp you could of bought a API test kit... Please buy water from your LFS. But always check the salinty of the jug, most stores have a habit of saying its 1.025 when really its more like 1.021... Saves them money... It does stink to spend money on not so fun things, but it makes the hobby so much more fun.
Water chemistry is the most crucial part of this hobby... Clownfish are so hardy those would probley be the last thing that dies in the tank... Coraline algae will die before a clownfish, I've seen it with my own eyes...
What types of corals do you have and how have they been acting lately?