It really is rather ironic. I am criticized frequently for my "hard line" on soft water and hard water fish species, because (as I get told in no uncertain terms) fish have been commercially raised for years and it doesn't matter. Now you are criticizing the only accurate site we have of its kind for allowing this with certain species. And if you are going to start criticizing, get your facts straight. It is certainly not accurate to say what has been posted by some in this thread.
To say something as ludicrous as "I have been looking at " Seriously Fish " every fish I have looked at will live in any tank that has fresh water, so please explain why test for anything. There is no point. " Seriously Fish " says it will survive in your tank." is inaccurate, misleading, and misinformed. There are dozens of species listed where the preferred parameters are either very soft, soft, or hard water. it clearly depends upon what species is being looked up. And I will explain why tests are important below.
The underlying issue here is the inherent preferences fish have for hardness, pH and temperature. Many but certainly not all species have very specific preferences, and the key to these is the natural habitat water. There are other species, quite a number, that generally speaking straddle the line between soft and hard water, slightly acidic to basic pH, and temperature. There are species that seem to manage outside the parameters one would expect (according to their habitat water), though it can be argued with intelligence that this is not as great as some assume.. This certainly does not apply to all species. SF is one of the very few sites that provides data that is reliable because it is scientifically-based. Very few aquarists have the knowledge level to even question such data. Experience does not make an aquarist unless it is based upon knowledge.
As for tests, these are important for beginning aquarists because they can alert you to potential issues and you can learn how to avoid them. Once you have the knowledge to recognize things yourself, they are less important. The majority of members on TFF do not have this level of knowledge, which is why they come here with questions and tanks of fish dying overnight. They can (hopefully) learn. I tested the pH and nitrate in all of my 8 tanks for years; first every week just prior to the water change, and on the day following, for several weeks, then sporadically for two or three years, then not at all. From the first the pH never varied by more than 2 or 3 decimal points in any of my tanks from test to test, and nitrate has always been in the 0-5ppm range. When you have achieved this level of success, you know you are doing things correctly. And the proof is that In 12 years I have only had three issues of disease where I had to intervene with so-called "treatments," and these came with new fish.