I'm just sayin. Just cause you cycled doesn't mean it wont again. Any thing could do it over cleaning the tank or filter. all the fish being sick at the same time says human error more than natural occurance. fish adapt to a wide rang of conditions wich is a good thing and a bad thing they will show decent health and then one day blam!
im actually considering breeding convicts/ kribensis for feeders once i get my fish into the 150. Sounds fun, almost --> (feeding baby fish that I raised, dunno if i could do it... lol)
but actually since you are having so much trouble with your 55 gallon, i wouldn't take media from it, but rather i'd go for the 10 gallon since i'm assuming it is disease free.