This is a hobby in which there are differing opinions on many issues. In many cases, there may be some degree of truth in the opinion. And in many cases the various methods/practices seem
to work. Many times in threads here I have read "well, I do this or that and my fish are OK, so there's no issue." But the problem is, how do you know that the fish are OK? External appearance is sometimes but not always indicative, and behaviours the same.
As a simple example, I often use the cardinal tetra. This fish occurs in water that is so soft the hardness cannot even be measured, and is very acidic (pH between 4 and 5). Many maintain this fish in water that is medium hard with a basic pH. The fish "appears" normal, it swims around, eats, and lives for 2 or 3 years. So why all the fuss over water parameters? Well, the scientific truth is, that this fish is not "OK" in such water. When maintained in very soft acidic water, it will live for more than 10 years. This is simply not possible in harder water, because of the affect of the hardness and pH on the fish's physiology. Among other things, it is prone to calcium blockage of the kidneys, and that kills it with no external signs. The calcium comes from the hard water, and the fish cannot handle it because nature did not design nor intend for this species to be in such water.
This is a scientific hobby. Fish are living creatures made the way nature made them. Science is only now discovering more about how they live. Many of the formerly-held "myths" are being exploded, but there are many who still hang on to those myths. The fact that there is sometimes no clear "right" and "wrong" means one has to take the macro as opposed to micro approach. By this I mean that we consider all aspects together, not individual issues in isolation, to get a better picture of what this or that may do. When we consider all the science, we are in a better position to see the individual effects. It is something like medications; medication A may cure "X" and medication B may cure "Y" but when combined at the same time medications A and B may kill the fish. Salt can cure certain parasites, and helps with nitrite to some extent. But continued long-term, it can kill the soft water fish.
A quick comment on stores. Some, sadly, are in the business of making money, and some staff are there to have a job. I was very fortunate when I started out in this hobby to have had a store owned by an aquarist who hired aquarists for his staff, and they all had experience in maintaining fish; that was a true blessing back in the days before the internet and forums like this one.
Just some ramblings that I hope may help somehow.