Phages are any bacterial, of protozoal agents that cause disease, (as said above), they are adapted by evolution to survive best in certain water parameters,
If you take a fish such as a oscar that has adapted to resist soft water pathogens (phages) and keep it in soft, with a low nitrate low pH water concentration, and keep it in a tank with that water , it has a good change of living a healthy life.
If you take that same oscar, and put it in hard, high pH water, with a high nitrate concentration (from last luster water changes), odds are not so good that it will lead a heathy existence over time.
Choosing fish that are adapted to your tap water parameters, will assure your success, but that means that doing a little research about the fish you choose, goes hand in hand.
If your tap water is hard, and has a high pH, choosing a cichlid from the hard, high pH water of either Central America, or the rift lakes of Africa will be a probably winning proposition (as long as you keep up with water changes).
But... choosing a Ram, or Uaru fernandezyepezi, (or Colombian oscar) will most likely be an exercise in futility over time (if you have hard, high pH tap water).
So....Just because you want a certain fish, doesn't mean your water conditions will allow it.
I have always wanted to keep Uaru fernandezyepezi , which prefers low pH (4) very soft, mineral free water.
My water in all the places I have lived), has always been around 8, and mineral rich.
So trying to keep that species, without significantly altering my tap water, would be best case scenario, delusional.
Over time, I have found that my best case scenario is to keep Central American, hard water, high pH speces, with the tap water I am dealt with.