1. Do you plan to have tropical fish or goldfish? (I personally recommend tropical fish, since goldfish are messy and they get too big for a 20 gallon)
2. Do you want to have real plants in your aquarium? Or just decorations and fake plants? Aquariums without plants are a lot easier to maintain, but real plants make the aquarium look better, and also provide fish with some extra source of food (algae and small organisms live on plants etc.)
3. What exactly species do you plan to have? If you don't know yet, answer this one first, what sizes of fish do you want to have? I personally prefer fish of under 3 inches long at full adult size, because I can have a lot of them then. I also prefer school fish, because it is cool to look at them swimming in groups together.
