Before you purchase anything for your aquarium, consider the type of fish you want to keep. Before jumping into the hobby and dumping potentially hundreds of dollars on equipment, commercial treatments and other necessities, it's best to understand exactly what your fish will need. A good place to begin deicding is with how large the fish will grow. A Betta that spans roughly three inches won't require anything larger than a ten gallon tank, and will likely not need a filter. But a Plecostomus that can grow 18 inches will need an aquarium with dimensions where it has enough room to turn in the aquarium, swim from one side to the other.
Next consider the Aquarium substrate. Bottom dwellers tend to need a softer substrate for them to lie on, while fish that are usually mid level swimmers can accept almost any kind of substrate. Bigger fish are likely to move the substrate, so a larger grain size will be needed. This discussion can also be attributed to plants, as different root sizes will grow better with different sizes of gravel.
Next consider the food your fish will need. Plecos can live off of algae Waffers and uneaten flake and pellet food from other fish, but they do also require driftwood, where they get their fiber and where naturally occurring algae will likely grow. Bettas on the other hand can require a wide range of foods. From simple pellets, to blood worms and Mysis.
Know if your fish will do fine on its own, or if it will require others of the same species. Keeping with our above examples, Bettas can be kept solitary. Plecos can be kept alone, but also in small groups. This will also factor into the size of the aquarium, as more fish will produce more waste and require more room to move. Schooling fish such as Angelfish, tetras and guppies will almost always require the presence of others in their own species, even if someone tells you they can live alone. This can mitigate stress factors.
Without considering and researching the needs of your fish, you can create a situation where a Betta, which is used to stagnant waters is being pushed around by too much flow, Plecos that are too big for the tanks that they are in, and schooling fish that maybe lonely, thus stressed out.
**Remember you can always upgrade your equipment later on. So receiving a young pleco of around 4 inches can be grown in a smaller aquarium, but will eventually need to be placed in a larger one.**