breeding rbps should be fairly easy as compare to other piranhas (I only aware of rbp and macs being bred in captivity), basically you just need to make them feel as comfortable as possible then do something trigger them to breed.
first you need to provide some space for them to build nesting site by remove some of the decors. might also want to add some "materials" for them to build their nest on. my old shola rbps just bred on gravel but I've heard some like to breed on java moss, or coconut fiber or some plant stems (after bite off the leaves) or even a piece of rock with some clean surface. I guess each p's has their own taste.
second try to simulate a dry/raining season as they breed in raining season. some people will go very extreme such as simulate rain fall and such but personally I've never done that, at least with rbps it might not be necessary. basically what I did was to hold off water change for a period of time and reduce feeding (or stop feeding) in the mean while to simulate dry season, (you can also lower the water level in your tank if you want to. if you plan to shut off the filter then make sure to add air stone to help provide surface movement for gas exchange.) then do larger volume water change with cold water then start the filter (good idea to run the filter in an other tank or at least put the bio media in a running take to keep all the bacterias alive so you don't have to worry about cycling the tank all over again) and increase feeding. this simulates raining season as rain fall will lower the water temperature and with new water being added the water parameter will get much better as compare to dry season. and food source will increase as well.
this had worked for me in the past so hopefully it'll help.
good luck with your breeding project.
