Rainbow Fish Not Eating

I just bought a maited pair of Rainbow Fish and one of them is not eating. The pH is fine and I keep the temp at 78.5. They look heathy and for now are buy themselves in a 20 gallon tank. Next week I intended to move them to a 29 gallon tank. Is a 29 gallon to small for one pair? Should I have More of them? I've fed them Flake food crushed, brine Shrimp and blood worms. What else can I try? Thanks for your help.