Doomguy
Tele-Holic
This isn't really true for farm raised fish/locally bred that are generations removed from the wild. Generally, farm raised fish and locally bred fish tend to thrive in the conditions that they were born and brought up in (with some exceptions). If you purchase fish from a place nearby you with the same pH, hardness, etc. as your water and they are thriving in it, they'll likely be fine. Of course you have to properly acclimate still.I've been out of the hobby for many years, but when I was a kid I always had a tank. One of the things I learned was that water PH and hardness compatibility was essential for the fish to thrive. Trying to mix species who do best in water with a lower PH with those who thrive in higher PH water will not end well. Same is true for relative water hardness or softness. Choose your fish species based on at least those factors. There are some other factors which include temperature, lighting and substrate which I found to be of relatively less significance, but are important considerations none the less.
Temperature has always been the most important thing to match IMO, as well as feeding habits. Food competition can be disastrous if you're not prepared to target feed less aggressive feeders.