Your temp should be 82F and Salinity on a refractometer 1.025.
That is the best possible reason they are dying.
2nd they eat as much a day as a fish their size, if you do not have enough food for them, there is no reason to have them.
No invert requires anything other than what's in the salt mixes already. If you need to dose, do a water change insted and you will get everything back that was consumed.