As said by others, your tap water seems to be soft, with very little buffering capacity (alkalinity).
Your public water provider can provide you the average values such as its pH, alkalinity , hardness etc.
It may be that you'd be better off with fish that live in your type water.
The origin of the FH is Amphilophus trimaculatus, and either Hericthys or Vieja specie all from hard, mineral rich, type water with pH around 8, found in wind swept area of Mexico where water temps are in the 70sF.
In soft water, bacteria that it has not evolved to resist in its natural conditions, may cause problems.
One of the double edged swords in your tank, is that this bacteria is often most virulent in temps in the 80sF (26'C and above), but because the FH is a hybrid, high water temps, and loads of antibiotics were used to increase production creating bacteria that are resistant to the normal antibiotics, resulting in disease and other bacterial and fungal maladies.
It could be, that your previous fish were resistant to those, but because you had to hastily combine the FH with them, exposed the new fish to soon, to those bacteria benign to your old fish, but oportunistall infective to the FH.
The purpose of quarantine is not only to protect old fish from the new, but also to slowly expose the new fish to low doses of those bacterial species, allowing for acclimatization with non-lethal doses.
Eating your tetras may have hastened that exposure.
Sorry to be so long winded,I was a water microbiologist, and it comes with the territory.