Mix up saltwater. Do live sand. Live Rock. Bingo. put emerald crab or 2 and some hermits in there and boom. g2g.
This from petsolutions.com
Scientific Name: Mithraculus sculptus
Ease of Care: Easy
Approximate Arrival Size: Medium: .75 inches; Large: 1+ inches
Native Region: Atlantic Ocean
Recommended Aquarium Size: 10+ Gallons
Temperament: Semi-Aggressive
Diet: Omnivore
Reef Safe: Yes
Emerald Crabs, Mithraculus sculptus, are a native from Atlantic reefs bordering the Americas. This crab is also known as a Mithrax, Clinging, or Green Crab. They are excellent scavengers of uneaten food, preferring to search primarily at night. The emerald crab will also consume Valonia and Ventricaria species of bubble algae. Emerald crabs make an excellent addition to the reef aquarium.
Notes: This species must molt in order to grow. Generally at night, it will lay on its back and exit its exoskeleton. A new exoskeleton is excreted and will harden over a period of several hours. The animal is vulnerable at this time and should not be disturbed. Proper iodine levels assist with successful molting.
I have two emerald crabs in a 40breeder and would not consider adding even one more. They are eating machines and live on algae. I can not imagine a 2.5 gallon being able to support even one.