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. Keep plenty of testing kits on hand, if you own a saltwater aquarium you should mainly focus on nitrates, PH, salinity, and phosphate levels. Never be caught off guard, make sure you have plenty of kits available to test the levels.
3. Goldfish can live incredibly long periods of up to 25 years. You should plan accordingly when you purchase a goldfish.