Yes you definitely need plants as they are part of the natural balance of a pond. I am definitely not an expert though but if you Google how plants help garden ponds, there are some explanations.
Now the list of plants can be long. What zone are you located in and what State? This will help determine what will thrive or die where you are located, i.e. tropical vs hardy plants. Also some States classify certain plants as invasive. Do you have more details of your pond setup such as filtration, waterfalls or not? Softliner I am assuming based on size. Are there any shelves built into the pond to hold plants?