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. 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.
3. If you have a protein skimmer you should shut it off when you are adding medicine to your aquarium. The protein skimmer will remove any medicines added to the aquarium.