There are two main ways in which fish are purchased: through a pet shop or retailer or through direct mail.
When receiving fish through the mail, it's important to turn off all lights and keep the fish from bright sunlight because they are coming from complete blackness. I set the bags into the tank and drape a towel over it to keep the daylight away from them. I keep the bags in the water for at least thirty minutes so the temperature begins to stabilize to the tank's water.
I then open the bags and wrap the plastic back in a roll so the bag floats on the top of the tank water (see picture above.) I do a water test to determine the Ph balance. Testing for nitrates, nitrites and ammonia isn't necessary because they will soon lose that water.
I use a turkey baster to suck up water from my tank and inject it into their open bag. I continue this process every fifteen minutes over a period of one-to-two hours until the Ph balance is identical to the water in which they'll be placed. This also helps with changes in water hardness, as many areas have softer or harder water than where the fish will soon end up.
Once they've become acclimated, I use a net to transfer them into the tank. I don't pour them in for one simple reason: I don't know if the tanks in which they originated had snails, and I don't want a snail problem.
I then remove the material that kept them in the dark but I don't turn the light on the tank for several more hours.
If I am purchasing fish from a local pet shop, I don't need to worry as much about water hardness. But I still use this system for angelfish, as they are more sensitive to water changes than many fish, such as tetras and bettas.
The picture in this blog today shows 11 baby angels shortly after I purchased them from a breeder roughly 600 miles away. They mailed the fish via overnight delivery. All the fish were received in healthy condition and they are now nearly a year old. Only one has had health issues unrelated to the shipping or environment.