My 29 gallon took longer than I ever thought it would, and I used Biospira. As my parents used to say- sometimes the magic works, sometimes it doesn't.
I have read that live foods and fish such as yours do create more waste by far than other types- which would explain the current problem. Adding both fish, and the additional waste all at once, even to a big tank, probably just overwhelmed the bacteria in the Biospira. Meaning, you didn't do anything wrong, just a bit fast for the bio to catch up.
All the advice I have is to do 20%ish water changes twice daily (if possible) and feed less. Fish can go weeks without food, so till Monday with very little food (or even none) and they will be fine. When you do the water changes, I recommend rinsing out the filter in the old water to get out any gunk that might have built up. I lost fish in my cycle because I did too much of a water change when the nitrites were scary high. So, small frequent changes seemed better. If I had done smaller water changes and not way overfed the fish in the first place, things would have worked out far better. Live and learn!
Once you return the Piranahs, I would recommend doing a water change but not necessarily the gravel. As you have gotten halfway through the cycle, I would say continue it and then it is done and good. You can continue it with either a super hearty fish, or fishless with fish food or
ammonia.
Good luck! One the tank has cycled, it will be worth it!
--R