The best thing to do is give them a bigger tank say a 180. Piranha pair off and spawn and protect there eggs so they will need a large tank so they can split off from the rest.
To condition them feed them well on lots of meaty foods like mussle, prawn, whitebait etc... and give them plenty of wood and plant cover. Id keep the water soft say a PH of 6.5. And you should be fine.
Where i work we had a pair of RBP in a 30" at " that would spawn most days, a greta fish to breed.