Rainford's Butterflyfish (Chaetodon rainfordi) are considered difficult to keep in captivity, particularly as they are obligate corallivores, meaning they feed primarily on live corals. They also require a large aquarium with ample rockwork and space for swimming. 
Here's a more detailed breakdown of Rainford's Butterflyfish care:
Tank Size and Setup: A minimum 500L (125 US gallons) aquarium is recommended, with plenty of live rock for grazing and hiding. 
Acclimation: Rainford's Butterflyfish are best acclimated to captivity when young. 
Diet: They primarily feed on coral polyps, but can also eat algae and benthic invertebrates. In captivity, offer a varied diet of meaty and herbivore foods, including frozen Mysis shrimp, brine shrimp, and freeze-dried foods with algae. Frequent feedings are crucial to prevent them from picking at corals. 
Tankmates: They are generally peaceful but can be territorial, especially towards food competitors and other members of their own species. It's best to avoid keeping multiple individuals of the same species in the same tank. 
Water Quality: Rainford's Butterflyfish are sensitive to changes in water parameters. Regular water testing and maintenance are essential. 
Temperature: Maintain a water temperature between 25-28°C (77-82°F). 
Salinity: Salinity should be kept at 1.023-1.025. 
Feeding: Offer a variety of foods, including frozen Mysis shrimp, brine shrimp, and freeze-dried foods with algae. Supplement with Brightwell's Angelixer or Garlic Power to encourage feeding.