I'm thinking you may have had a mini cycle since your not reading any Nitrates.
When you mentioned new fish hitting glass...was it darting or a light tap ? If darting/spooked...it's possible cause maybe just hit hard enough that killed them.
Cloudy water could be the cause of the "break in cycle" or mini cycle I mentioned already. Usually it clears after the bacteria catches up.
As for the zit...either Ich or just poor water quality. Hard to tell without photos.
...Ralph