I was just thinking about this today. I've had some issues with a few female guppies not living more than 2 weeks after purchase. No visible reasons... and it has been when purchasing two.. one lives.. the other doesn't. My water stats are fine 0, 0, 40. My Nitrates have always been this number. Tank is established and up and running for 7 months now. All my other guppies that I've had or raised from fry are fine. So I got to wondering about if the ph or nitrate levels were soooo different at the LFS. Introducing my fish has always been the standard float and then add water every 15 minutes. So that is not it either. I also know that it is so hit and miss these days with fish being more weak than what they used to be. I will not be able to get to my LFS until next Sunday.. but I'm testing their water somehow. lol Could this be an issue when purchasing new fish?