First off, I’m sorry for your losses!
Second, I’d guess disease or fish-on-fish aggression before assuming the shrimp are the problem.
Some questions to help find the issue here:
-Is the tank cycled? (Numbers for as many parameters as you can list would be helpful.)
-Are you quarantining/buying pre-QT’d fish? If not, what are you doing for disease control?
-What animals do you have the tank (you mentioned the two shrimp, but do you have anything else in there too)?
-Have you observed any signs of disease or aggression?
-(Edit: ) How long has the tank been running? Any pics of the tank?
And a couple of quick notes:
-Tangs and Chromis are both known for being relatively susceptible to disease.
-Chromis are known to whittle away at (i.e. kill off) other chromis in our tanks despite being a schooling fish; long story short, for many fish, schooling is a relatively complex behavior (see the quote below), and they don’t always appreciate having other fish of their same species around in our aquariums. (I’ve heard that some fish - like some Dartfish species - are peaceful and will school/shoal without issues, but fish like chromis seem to need some motivation to accept/engage in schooling.)
-Tangs are a definite “No” for long term inhabitants of a 35 gallon cube (the tank is way too small for a grown tang - it could work temporarily if the tang is tiny like yours was, but they will quickly outgrow it and need rehomed; personally, I’d just say no tangs in that tank); you might be able to get away with the chromis (I’ll defer to more experienced reefers there), but with less than two feet of swimming space, that’s probably pretty cramped for them.
The quote mentioned above:
My understanding is that keeping a large school can theoretically work, but - as mentioned above - there are a number of things to keep in mind with schooling:
- The number of schooling fish in the tank (I've heard odd numbers are preferred, and the preferred number of fish that I've seen seems to be 9 to 11 at a minimum; the more fish, the more diluted the aggression is between them)
- The size of the tank (for quality of life purposes for your fish, bigger is better - yes, some animals will be calm when shoved into tiny spaces with large numbers of conspecifics where if there was just one or two conspecifics, it would be a deathmatch, but they obviously wouldn't be happy in that situation long-term)
- The amount fed, the quality of the feed, and the frequency of feedings (basically more food = less aggression; and better food = better health = happier, less aggressive fish [theoretically])
- Tankmates (big, scary tankmates that the schooling fish could view as a threat may act as an outside force that keeps the schooling fish focused on not getting killed rather than on fighting amongst themselves)
- The scape of the tank (lots of fish need lots of places to hide/sleep - the more hiding places, the safer the fish feel; line-of-sight-breaks can also help with feeling safe)
That's all I've got for the moment (and pretty much all of it has been mentioned above), but basically - to my understanding - it's a balance of making the fish feel threatened enough by external sources to prevent infighting while also making them feel safe enough (largely through their numbers, the tank's scape, and food security) to not be too stressed. If you're able to strike that balance, you should be able to see schooling behavior (to the best of my current understanding).
