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. You should keep plenty of water testing kits for your aquarium. Testing kits can measure the levels of harmful and beneficial chemicals in the aquarium. You will need these testing kits for the frequent water changes for 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.