1. It is generally a good practice to purchase the largest aquarium that you can actually afford. A larger aquarium will be less susceptible to quick changes in the chemistry, and will offer much more stability.
2. Always use a new or clean bucket when going through the acclimation or water changing process. A bucket that has been used to hold bleach or other chemicals can be very harmful for the fish in your aquarium.
3. Catfish sometimes require freshwater as juveniles and then as they get old they will need a more brackish environment. Make sure you read up on your catfish to learn what its needs will be when it gets older.