Mysterious Fish Disapearances

[Jenn_737700]

Lately, every time I get a new fish, it almost always disappears in the next few days. Im pretty sure that it isnt my fault, like, it didnt die or anything. Here are my levels:
ph: 8.2
amm, nitri, and nitra all at 0.
Temp:79
s.g: 1.025

I bought a royal dottyback around a month ago, but she dissapeared. So did the firefish I bought 3 weeks after that. Then, two days ago I bought both a royal gramma and a neon goby. The royal gramma is gone, but the goby is perfectly fine. I also have two clownfish, and a peppermint shrimp and they're fine as well.

The suspect I would think to be the culprit might be a hungry hitchhiker crab... other than that...its a mystery