1. 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.
2. Only use medication in your aquarium when you are out of other options. You should use a separate tank to medicate sick fish as some medications can have harmful effects on healthy fish.
3. 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.