"So this is are new home Peaches?" "Oh yes, I think I see a flowerpot down there.....safe..I hope"
Perhaps this is the most crucial time for a fish that will live and thrive in a aquarists aquarium(say that ten times fast). Many beginners make the mistake of quickly netting the fish and dropping it into the tank, or simply dropping in the bag and "letting them swim out by themselves". Or picking unintentionally a sick individual, or assuming that a healthy individual in a tank of sick fishes is okay. All this may result in problems immediately(fish stressing out and dying) or later(diseases introduced into tank). Here are some tips on helping your new purchases live to become established as part of your tank.
When you buy a fish from the fish store you should be strict when it comes to choosing. Look around the stores tanks. Look at the fish and check to be sure that they look happy. Be wary of a shop in which there are dead fish floating in the tanks or sticking to the filter intakes. Check to be sure that the fish in each tank or compatible. Don't single out a fish from a sick tank, as the trip home will stress it and then it may well fall victim to the disease its pals were already infested with. Be particularly careful if you are buying WC fish, and other fish that have weak genetics or may have been exposed to bad farming methods(neon tetras and German blue rams). When the employee comes to get you your fish, be sure you know which fish you want. Its best to keep your eyeball on a certain fish that you like and be able to tell it apart from the others. I usually spend several minutes inspecting the fish in the tank. If your fish has a distinctive coloration or marking that distinguishes it apart from the others, memorize it. Before the employee even is gonna net the fish for you, check for all signs of disease, like white spots, clamped fins,fungus.extensively damaged fins(a nipe or to may be okay) listlessness,(the little pale white african cichlid, cowering at the corner with lots of torn fins is not a good choice). With some fish(such as tetras and danios) it is useless to try to catch those several fish you desire, so check the whole tank and know what you might get. Also be sure that the employee does not net any old, sick, or bent backed fishes. Be sure you know what species you're getting, and if its important and/or possible, know the sex. The employee will net your fishe(s) and then bag them for you. For a long trip, it may be a good idea to the desk and see if you can get pure oxygen for the trip. Pressurized air is the second best choice. Some places just blow up the bags(not blowing into). The fish should be double bagged in case the first bag leaks. If you are buying a fish that has the potential to burst or cut the bag, bring along a jar or buket for containing it. If you are purchasing aquatic snails, bring a small margarien tub withdamp paper towels, into which the snails are imbedded to prevent rolling.
When ordering online or trading with other hobbyists one does not have the ability to check the condition of the fish involved. You simply have to be able to trust the person on the other end of the line to provide you with good stock.
A car has a chance of being exposed to many situations that can be lethal ot your newly acquired treasures. If you can resist the temptation of looking at them during the drive(yah..I admit, very hard to resist or), put them into a paper bag and keep this bag dark(less stressful). After you buy the fish, go straight home, and don't take a little trip to the mall or anything. A fish left in the car can easily be broiled in the car, or turn into a floating fishsicle. For particularly large and/or sensitive specimens, you should pack them in a styrofoam box. Fish can also ride in an icebox(iceless of course). Be sure that the bag(s) do not have the ability to roll, further stressing the stressed occupants.
When your treasure arrives at home, remove its bag from whatever it has been stored in. Undo the rubberband, and then remove the first bag(you can use it for other things, such as taking your fish back to the store). Float the second one in the tank. Wait till the temperatures are about the same, and then open the bag. Remove some water form the bag and dump it down the drain. Do NOT add the bag water to your tank(obvious reasons). Then, replace that water with some of your new water. Take another tne minutes before repeating this. If the conditions are relatively the same, you can release the fish after 10-30 minutes. If you are dealing with a delicate fish or one from a shop with markedly different ph hardness etc. , then you should take more time, perhaps an hour. Afterwards you can either net the fish with a small net, or remove it with your hands(this works well if the bag is to narrow for the net to fit it, or if the fish is apt to injure itself in the net). Special nets for fish capable of getting stuck in the normal ones(catfish, some cichlids) are available and should be used, lest your fish get caught and left to dangle helplessly to the net by one of its spines. A quarantine tank is a useful precaution, and many aquarists will keep the fish in this tank for some time ot be sure its not sick before adding it to the tank with everyone else. Add fish to tank, and let it swim off to hide. Feed occupants of the tank if there are any, and then switch off the light. Don't expect the fish to eat on the first day, although it may...but expect it to hide. Some fish, that routinely carry parasites, such as rams, will need to be medicated before being added to the tank(read: Q tank).It will come out after several days. And when it knows who is the food dispenser, your new occupant will be established into your tank!
Receiving fish shipped through the mail is similar, but if the bag shipped is too small than it is helpful to empty the fish into something like a yogurt tub to float. With shy fish a handful of plants added to this container could help lessen the stress of being trapped in a bare space. I hearl that you should not float breather bags as the fish inside may suffocate but have yet to verify this.
Good Luck to you and your fish!