Let me help you out with this information from Seriouslyfish about rainbow sharks:
Maximum Standard Length
130 – 150 mm.
that is 5.9 inches not counting the tail
Aquarium Size
An aquarium with base measurements of
48" ∗ 18" should be the smallest considered for a single specimen, with larger quarters necessary for a group.
Diet
Primarily an aufwuchs grazer feeding on algae, small crustaceans, insect larvae, etc., and for it to develop its best colours and condition it should be offered regular meals of small live and frozen foods such as chironomid larvae (bloodworm),
Daphnia and
Artemia along with good quality dried flakes, granules and fresh plant material.
Shelled peas, cucumber, blanched courgette, spinach and chopped fruit all make good additions to the menu. Once settled it will often ascend into midwater to feed and browse the biofilm that tends to form on rockwork and other solid surfaces.
Behaviour and Compatibility
While small specimens tend to hide away much of the time they become increasingly territorial as they grow and can display particularly high levels of aggression towards similar-looking species........
This species probably lives a solitary lifestyle and in nature would probably have only come into contact with others of its own kind infrequently and during the spawning season. These instincts heighten as the fish get older and we therefore recommend it be kept singly in the majority of cases. In a very large tank with lots of cover a cohabitation attempt might be possible but each individual is likely to require a territory with a diameter of at least a metre.