So I have a relatively new (about 1-2 months old) 10 gal saltwater tank. I had a percula clown, a pajama cardinal, 2 hermit crabs, 2 turbo snails, and a peppermint shrimp. The day before yesterday one of the snails died so I took it out, and yesterday I acclimated a red flasher wrasse. I noticed last night that my pajama cardinal was hanging out behind some of the liverock, so close to the bottom that its fins were touching the sand. It stayed there for a while, but after I shut off the lights I saw it swimming. This morning, it's gone. I notice that my scavengers are all gathered in the same area, and I can't be sure but it looks like there's something white down there that they're eating - I assume it's what's left of the cardinal. Can someone give me some insight onto why it might've died? The clown and wrasse seem super happy (the wrasse seems even more active and happy than he did in the store and the clown is by far my friendliest and most active thing in there) so I was wondering if maybe the wrasse wasn't compatible with the cardinal? I'd just like to know for next time so I can avoid a repeat of this. EDIT: I got my water tested and the pH is too low + the nitrates too high :/ the store I usually go to wasn't very helpful so I didn't know how to fix it, but I went to a different store and they're helping me. I got buffers to add to the water, and hopefully in a day or 2 I'll get some water to do a water change.