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. Make sure to always keep your aquarium level when placing it. If your aquarium is uneven, the weight of the water can press against the seems and this can cause major problems in your aquarium.