Aquarium Murder Mystery (Help me save my fish please!)

Hacim03

Hey ya'll,
My newly added hawkfish just died and I'm pretty devasted, this isn't the first time either, I've been having issues keeping newly added fish and inverts alive (especially those that live in the rockwork). So if you all could be most kind and put on your Poirot, Columbo, Monk, Holmes, Marple, (Or favorite detective of choice)'s hat and help me find out who is most likely responsible for these weird deaths.
Timeline:
Bought an already running tank, which came with fish (I sold them) but kept the hermit crab that came with ( 2", so pretty decent size)

Since I already had a large population of beneficial bacteria, I go and buy 2 Ocellaris clowns, Pistol shrimp, Yellow watchman Goby (not paired), Emerald Crab, and Camel Shrimp.

3 weeks later I find one clown staying in a corner, with a whitish belly and nipped fins (completely out of the blue) and dies that night, I assume that one clown didn't like the other and offed him.

1 week later I buy another clown (to see if those 2 just had compatibility issues) and Peppermint Shrimp, within 2 days Clown dies in the same way as other but takes longer.

3 weeks later I go buy Arrow Crab, and Royal Gramma

2 days later gramma flails wildly within the tank, mouth stuck open, and dies. (Sister said she saw arrow crab snap at him, but is unsure)

Got sick of buying fish and them dying so don't buy anything for 6-7 weeks, I then buy 1 Bangaii Cardinal and Chocolate Chip Starfish (both still alive and quite well).

1 week later and Arrow crab dies, (but there was a nitrite spike at the time so I blame that)

3 weeks later and buy Coral Beauty and 3 chromis (CB still alive and quite well, but has a small cut on it's fin, 2 chromis doing great, but one was hiding and had like dark spots on its head, dead within a day after I moved to QT tank, thought it was uronema or chromis bullying).

3 weeks later I buy a Red burrowing crab, Feather Duster and a Rose pink Urchin

5 weeks later I buy a One Spot Foxface and a Lawnmower blenny

Around 3 weeks later small pieces of red burrowing crab are found throughout the aquarium (but might have been molting not sure my brothers told me I was out on vacation)

2 weeks later I'm home and go buy a decorator crab, fighting conch and Flame Hawkfish, that day before even adding fish Lawnmower Blenny dies, their belly looks a little thin. no previous signs of downward behavior, exactly like all the other fish, large hermit at the scene of the crime biting at him cuts on his fins but not sure if thats scavengers .

2 days later Hawkfish dies, their belly was a little white and the hermit crab again was found at the scene of the crime.

So there it is folks.

The suspects are these

Tiger Pistol Shrimp: Likes to snap at things that come near their cave and constantly does this. never seen it attack anything and seems to just scare off intruders

Yellow Watchman Goby: Have seen him bite at fish and inflate his mouth but that is a very rare occurrence. is territorial and did sometimes chase lawnmower blenny.

Bangaii Cardinal: Just swims out in the open, doesn't do much, doubt it's the killer.

Chromis: Like to pick on each other but never have gone after other fish.

Hermit Crab: always seen at the scene of the crime "cleaning up" but seems pretty peaceful.

One spot Foxface: Very Docile, but raises it spines when fish swim near.

Coral Beauty: pretty much oblivious to everything else in the tank, besides the Foxface who he swims with. has a small cut on analfin.

Clownfish: Really, Really small, barely an inch, but is suspected of killing their mates, but ever since mates died, sleeps at the top right by Hob filter.

Spider decorater Crab: slow and just got, might be doing something, although he seems too lazy.

Emerald Crab: He is like dime-sized and I know, he's a he so might be aggressive although I've never seen him do anything against other fish or inverts.

Disease: this is a good possibility except that fish seem to have been attacked instead of sick, (due to a lot of small, rock dwelling fish and inverts dying)

Bad Water quality: I don't think it's this one either because I change water and test water consistently and always is good.

Any help would be literally a godsend, cause this has given me and my family so much heartbreak (the flame hawkfish and the lawnmower blenny were my mom's and brothers favorite fish respectively).
Thanks so much for reading this long post!

PS.
forgot to add a few things,
almost all the fish that survive are the ones who swim in the upper or middle part of the tank,
I have Live Rock, so maybe a bad hitchhiker keeps knocking my fish off,
I have seen the yellow watchman goby grab a small hermit crab and take it to its cave.
I also got a Neon goby, bought with CB and had for 2 weeks and liked to hang out by the silicone siding near the bottom, disappeared, assumed he jumped because I had some holes in my Lid.
 

Sunrise

Are you quarantining your fish before adding them to the tank? Also there might be something in the rocks I’ve seen reports about how peoples substrate gets a buildup of toxic gasses, then when someone moves it it all releases, killing the fish. That’s just speculation.
 

Hacim03

I just started qt (stupid beginner move, I know), but pretty sure isn't a disease, Hawkfish are fairly resistant to most diseases, and it succumbs to it in a night doesn't make much sense, when the foxface and angel are doing great after 2-3 months. I settled on it being an insanely aggressive YW Goby, I've seen him attack nearly everything in the tank. Putting him in a tank by his lonesome with the pistol shrimp. I've recently moved around my substrate and everything was fine so I don't think that is it either, plus I've had the tank not too long.
Thx!
 

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