Colder temps can make fish less active. I keep my red bellies around 70° during winter so their less active thus eat less but when I crank the temp back to 78° they are much much more active and hungry.
Also since its already 6" you should start practising big feedings, such as 1 large feeding every 4 days or so. Good for them to be hungry
As said above wait a week or 10 days then offer talapia or pellets. Personally I would hold out for pellets such as NLS and you will see colors pop, at that point supplementing talapia is always a good idea.
One other idea, high lighting can make piranhas skittish or want to hide (less active), just something to consider since their eyes are huge.
-GO S. VETTEL #5 SF15-T!!!
POTHOS:
http://www.monsterfishkeepers.com/f...ts-less-nitrate!-POTHOS&p=6044042#post6044042