When you take your new acquisition home, remember again: patience! Open the bag and float it in the water (without letting the guppy out!). Secure it to the side of the tank with a clip away from lights or the heater. Add a small amount of water from the tank into the bag, and let sit for 10 minutes. Then remove some water from the bag, pouring it down the sink - NOT into the tank - and add some more water from the tank to the bag. Continue this process every 15 minutes or so until all the water from the bag is tank water, and let the fish swim out on its own. This is the only proper way to acclimatize new fish, and it should take almost an hour.
Test the pH of your water and the water from the store or breeder you want to purchase from. If there is a difference of .3 units, there will be undue stress for the guppy in acclimatizing. A change of .3 per day is stressful. If you must purchase from a tank with a large difference in pH, gather some of the store's water for a quarantine tank, and slowly add some of your water until the fish has become accustomed to your pH.
Some guppies will be used to a salted environment, particularly those that come from breeders, and if you are spending a lot of money on such quality fish, you should consult with the breeder and see if he uses salt; if he does and you do not, you can have a lot of trouble keeping his fish alive. A fish going from a non-salted tank to a salted tank is fine, but a fish used to a brackish tank should remain in such an environment.
Remember, no matter where a fish comes from, it undergoes a stressful change in it's life when it is moved. It's immune system is affected adversely and every precaution to reduce stress should be taken.
