Red-legged hermit crabs, 2 arrowhead crabs (hardly seen, I'm thinking rectangulus trigger may have eaten them). Yeah, I mean the fins are ripped
If the arrow crabs are still in there, they may very well be the culprits. Quote below taken from LA about them:
http://www.liveaquaria.com/product/prod_display.cfm?c=497+501+616&pcatid=616
This crab has been known to attack crustaceans such as Banded Coral Shrimp, and may even attack small slow moving fish.
I also wouldn't put it past the trigger, but you should see body wounds. Of course, a disease is always possible but impossible to say without seeing symptoms. The next time a fish dies drop the body in FW to check for flukes (see below).
Freshwater Dip: Provides temporary relief for Brooklynella, Flukes, Marine Velvet disease (Amyloodinium); possibly even Ich & Uronema marinum (both unproven). Can be used to confirm the presence of Flukes.
How To Treat - Fill a bucket with RODI water, and use a heater to match the temperature to the water the fish is coming from. Aerate the water heavily for at least 30 minutes prior to doing the dip, then discontinue aeration while performing the dip. Fish aren’t overly pH sensitive for short durations like this, but you can squirt a little tank water into the dip just before the fish goes in to help bring it up.
Place the fish in the freshwater (FW) dip and observe closely. It is not unusual for them to freak out a little at first. Also, tangs are notorious for “playing dead” during a FW dip. The important thing is to watch their gills; they should be breathing heavily at all times during the dip. If breathing slows, it’s time to exit the dip. Dip the fish for no longer than 5 minutes. Multiple dips may be done, but it’s important to give your fish a day to recuperate in-between dips.
For flukes, use a dark (preferably black) bucket so you can see if tiny white worms fall out of the fish (especially out of the gills) at around the 3-4 minute mark. The worms will settle to the bottom, so you can use a flashlight to look for them there as well.
Pros - Provides temporary relief for a wide range of diseases in a chemical free environment. Can “buy you more time” until a proper treatment can be done.
Cons/Side Effects - Not a permanent “fix” for any disease, as FW dips are not potent enough to eradicate
all of the parasites/worms afflicting the fish. Some fish can have an adverse reaction to a FW dip by appearing unable to maintain their equilibrium once returned to the aquarium. If this happens, hold the fish upright (using latex, nitrile or rubber gloves), and gently glide him through the water (to get saltwater flowing through the gills again). It is also a good idea to place the fish in an acclimation box until he appears “normal”.
