doing the water changes when you are is a good idea, and probably the best advise to offer at your current spot. But your nitrites could sneak up on you, So def invest in a test kit, and if you can't take it to a local LFS and have them test it to make sure everything is running as it should be.
I don't know alot about Piranhas but the sub-forum may also help with any chem questions. I do vaguely remeber hearing blacks can be delicate to water. So keeping your ammonia low is likely your best call, but doing this may also lengthen your over-all cycle process. alot of people reccomend and use Seachem Stability for bacteria in a bottle. personally I don't trust anything in a bottle, thats suppose to be alive. But you may look into it to help speed up your cycle processs. I've never heard of it hurting a cycle reguardless if ti doesn't work. ( and sometimes you just get a bad batch ect)