Plants
Moss: Moss isn’t just placed in a fish tank to look pretty. It’s known to better the quality of water and provide oxygen for the fish. Aquatic mosses absorb substances in the water, such as nitrates, and act like a natural filtration system. These nitrates can be harmful for the fish so it’s good to keep some moss present in your aquarium. (By Ian Bacchus, Aiden Benoualid, Davis Lail, and Max Stiles)
Anubias: Anubias is a plant genus that includes many freshwater species. They grow slowly and do best in water that is 26 – 28 degrees Celsius with 11 – 12 hours of light per day. They are native to tropical areas and western Africa. (By Arden Koch)
Artwork of Anubias by Alana Hennon:

Water onion: Water onions are an aquatic plant that are originally native to Thailand. The leaves of a water onion grow in a flattened fan-shape. Average height is between 15 to 45 cm (6 to 18 in). (By Anna Bourassa)
Artwork of water onion by Alana Hennon:

Aponogeton: Aponogeton is an aquatic plant with long, flowing green leaves and small, white flowers. It comes in very small bulbs, but they grow very quickly to considerable sizes, up to 12 inches for each leaf; it’s best to plant them in at least ten- gallon tanks with nutrient-rich substrate.
Photo of Aponogeton (thanks, Pixabay):

Invertebrates
Ramshorn snails: Ramshorn snails are unique in quite a few ways; for one, their shells are flat spirals, rather than curving to a point on either side. Ramshorn snails are often introduced to tanks by accident and multiply quickly. However, they are low-maintenance and peaceful, and they can easily eat any algae in the tank and leftover pellets and food. (By Brady Greene and Malysha Winston)