It’s really unlikely. What about cooking? Did someone cook in oil or something else that could create smoke or airborn toxins? Were the tanks with issues close together or far apart? Near the kitchen (or other common room)? Near open windows? What do the affected tanks have in common that isn’t shared with the unaffected tanks? Location? Hands in tanks? Treatments (chemiclean, Nopox, etc.)?Can contamination occur from the shells or the little water that transferred with the animals to the acclimation bucket and then to the tanks?
For ammonia we use API. But the nyos nitrate test is at 0.06 which is about what we normally have.
Also our salt water is all mixed using RODI which regularly tests at 0-1 tds.
I don’t think stray voltage could affect multiple tanks at the samr