The Fish Killer

Why am I the fish killer? I've had my reef tank set up since spring 2015. It's a 20 gallon tall w/ liverock, sand bed. I have had several snails and a skunk cleaner shrimp since August 2015. At that time I also bought a clownfish and a royal gramma. These came from PetCo. Both perished within a couple of months. The shrimp and snails continued on. I waited until this summer to add another fish, another royal gramma, and it, too, has died. The hermit crabs and snails I bought at the same time as the gramma are still going strong, as is my shrimp and original snails. I'm not sure what I am doing wrong! I have two small coral frags-- a green star and a pink polyp of some type. They aren't exactly thriving, but they aren't dying either. But it seems every fish that goes in dies.

My ammonia, nitrite and nitrate are all normal (0), my pH = 8.3 and my salinity is about 1.024. All of these seem to be within normal range for fish. Can anyone help me with this? The tank is in my HS science classroom and I'd like to have fish in it.