Answer: One possible ecosystem in an aquarium is a freshwater tropical community.
Abiotic factors:
Water: The water should be clean, filtered, and aerated. The temperature should be between 24°C and 28°C. The pH should be between 6.5 and 7.5. The hardness should be between 4 and 12 dGH.
Light: The light should be bright enough to support plant growth, but not too intense to cause algae growth. The light cycle should be 12 hours on and 12 hours off.
Substrate: The substrate should be fine gravel or sand, with some larger rocks or driftwood for decoration and hiding places.
Plants: The plants should be live aquatic plants that can tolerate the water conditions and provide oxygen, shelter, and food for the animals. Some examples are java fern, anubias, amazon sword, hornwort, and water wisteria.
Biotic factors:
Fish: The fish should be compatible with each other and the water conditions. They should have different sizes, shapes, colors, and behaviors to create diversity and interest. Some examples are neon tetra, guppy, platy, corydoras catfish, and angelfish.
Snails: The snails should help control algae growth and scavenge leftover food. They should also add some variety and movement to the aquarium. Some examples are nerite snail, mystery snail, and ramshorn snail.
Shrimp: The shrimp should also help control algae growth and scavenge leftover food. They should also provide some color and activity to the aquarium. Some examples are cherry shrimp, amano shrimp, and ghost shrimp.
Niche of two species:
Neon tetra: The neon tetra is a small, colorful fish that lives in the middle layer of the water column. It is a schooling fish that prefers to swim in groups of six or more. It is an omnivore that feeds on small insects, worms, crustaceans, and plant matter. It is a peaceful fish that can coexist with other community fish of similar size and temperament.
Corydoras catfish: The Corydoras catfish is a small, bottom-dwelling fish that has whisker-like barbels around its mouth. It is a shoaling fish that likes to stay in groups of three or more. It is a scavenger that feeds on leftover food, algae, and detritus. It is a peaceful fish that can coexist with other community fish of similar size and temperament.
Hope this helps, and have a great day! =)