You should never have uneaten food in pygo shoal tank you've had for a year +! (Unless experimenting with types or foods)
Amano shrimp & cardinals are just an expensive meal IMO. If the cons breed in that tank the piranhas will slowly pic off the fish but most assuredly the adult cons will be dinner(or too stressed to breed), it may not be today, next week, this month or even this year! All it takes is for some nosey piranha to spark interest and before you know it the whole shoal will stalk and kill! IMO there are no tank mates with pygo shoals, a mate is a fish that can live together and with piranhas that's just not possible. Yea some have kept random fish with shoals but eventually they will get eaten or terrorized so bad they wish they were dead and that is by no means a way of life...
With that being said I keep guppies in all my P tanks just because their small, 7 cents and they breed like crazy! Plus they always scavenge the crap that's on the sand, pellet waste is perhaps the worst! Glass shrimp work awesome as a cleanup crew/occasional snack as well but where I'm from amano shrimp are like 3-4$ a pop so I just stick with ghost shrimp. These combined will not only clean your tank but also act as random snacks for the shoal and there isn't any remains left over! Buy about 2 dozen guppies from a reputable lfs and throw them in, they will breed in no time and they will act as a healthy food source because they will eat the left over pellet crap and the piranhas only digest 50% of the nutrients when they eat (like all fish) so the pellet crap is a decent food supply!
Or get an auto feeder and calibrate it to drop 5 or so pellets per fish when your away..... I've had good luck with this method but after much research and experience adult Pygos can easily go weeks without food anyways, also since their adults their metabolism is slower meaning they don't burn as much energy, equating to less frequent feedings!
My adult shoal gets white fish fillet chunks every Sunday and pellets(NLS) on Weds or Thurs & they thrive! Hungry fish is a healthy fish and with extra food after feeding you may want to cut down on the amount of feedings! I feed a huge fillet every week and you would never think my shoal could eat that much food but they do because their hungry!
https://www.youtube.com/watch?v=pMlTNLAYG8E&feature=youtube_gdata_player
Just some advice!
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