Since your tank is so new I'd be testing for ammonia - and right now
and probably would be doing big (25-50%) WCs if you can't test
If you need to buy an ammonia test kit and some ammonia binder stuff, such as ammo lock or something similar, so be it
Test for ammonia
With a new tank, you need to monitor your NH3, NO2 and NO3 during the cycle
During the 'whole' cycle you can count on losing critters, so you don't add any
Why did you add so many so soon ?
Once the cycle is complete, you add 1 or 2 critters
Then, when that cycle has gone around again, you add 1 or 2 more
I wouldn't have added fish so soon
and would have stuck with crabs/snails to start with