If you choose to add more fish, feed the old fish first and then turn off the lights. When you add the new ones, the old ones will probably be too full to bother them and may not even notice their arrival in the dark.

2torial #0705: Learn2 Set up a Freshwater Aquarium Add your fish When you go back to the pet or aquarium store to pick up your fish (and don't forget their food!), buy fewer than your tank can support. Even with good filtration, too many fish in a newly prepared tank can overload it with toxic bacteria and ammonia. The tank needs time to adapt, so pace yourself. The store will put your fish in a plastic bag for transport. If the trip from the store to your home is more than half an hour, leave some oxygen in the bag for safety. When you get home, make sure the bag's water temperature matches the tank's to avoid shocking the fish. You can float the bag in the tank water to even the temperature, or open the bag and add small amounts of tank water to it. When the water is ready, open the bag gently so the top is underwater and the fish can simply swim into the tank (don't pour them in). You also won't need to feed them their first day, because they'll probably be too stressed to eat. Even if the fish are swimming and the equipment is running, keep a close watch on everything. Check the equipment regularly (especially the filter), keep an eye out for any signs of fish stress (gasping for breath, loss of appetite, fungus or parasites), and don't forget the monthly water change. As your fish settle in, you'll become familiar with their individual and species habits, get to know their likes and dislikes, and be an integral part of their underwater world. -end- Learn More! or All steps at once (printable version)