What happens next?

How to acclimate your fish to a new aquarium
Use this method unless you know the place you got it from uses best practices, especially if you have a saltwater tank.
The problem is the disease or parasites are in the water and on the fish. Even if you transfer the fish without the water into your tank, the fish can still carry parasites and disease.

Options:
(a). This can make it easier for the fish to breed and feed. Be aware that this method doesn't cost as much as the other methods, but places containing a aquarium can sell it for a decent price.
(b). If you don't care for the fish, there is a fine line between being too reluctant to take the time to fully care for them and being too trusting. Make sure the tank isn't too cold.
(c). Without putting the fish into quarantine you really have no idea what you are introducing into your tank. Unless you treat the fish first, or know for sure the store has, you are risking your entire tank.
(d). Consider getting your bag of aquatic plants in with the tank. Plants can act as allergens so it's important that they get good air circulation.
(c).