I tend to find the best way to introduce new fish are as mentioned if possible use a QT tank (quarantine) to avoid introducing a desires into the main tank, if you cant do this then keep an eye on all fish in the main tank.
To introduce them I 1st turn the lights off on the main tank, put the fish bag containing in the new fish into the tank and let it float for about 15 mins so the water the new fish is in becomes the same temp as your tank, i then open the fish bag after 15 mins and fill the bag to 3/4 full with my tank water and leave the bag and fish bobbing around for a further 15 - 20 mins, after this time i lift the bag with the fish in out of the tank, get a little 5ltr bucket / tub and pour the contents of the fish bag into the bucket THROUGH A FISH NET !!!!!! so the fish is caught in the net, i then put the fish in the main tank and leave the lights off till the next day. the reason i put the water from the bag into the bucket and not the tank is just in case the water from the ls is not good, so i don't contaminate the tank.
To introduce them I 1st turn the lights off on the main tank, put the fish bag containing in the new fish into the tank and let it float for about 15 mins so the water the new fish is in becomes the same temp as your tank, i then open the fish bag after 15 mins and fill the bag to 3/4 full with my tank water and leave the bag and fish bobbing around for a further 15 - 20 mins, after this time i lift the bag with the fish in out of the tank, get a little 5ltr bucket / tub and pour the contents of the fish bag into the bucket THROUGH A FISH NET !!!!!! so the fish is caught in the net, i then put the fish in the main tank and leave the lights off till the next day. the reason i put the water from the bag into the bucket and not the tank is just in case the water from the ls is not good, so i don't contaminate the tank.