That's not true (unless you mean qt as husbandry). A fish doesn't get sick from velvet or similar because of poor quality water or care. They get sick because they get in contact with it and the disease kills them. It's sorta like saying that Marburg hemorrhaging virus only happens / kills people because they aren't eating a good diet.
We will need to agree to disagree on this one. We have incompatible opinions regarding this particular issue.
Disease preys upon weak and fragile, not the strong and robust.