convicts

Ladies & Gentemen,
Today I have performed the impossible!
I've managed to kill two adult Convicts by moving them out of a 30gal tank
to a 55gal tank with the same water parameters.
Reason for the move, breeding their tits off, sick off trying to capture
little dudes when they become Oscar fodder, sounds horrid, but heh!
Anyway did weekly water change on both tanks, as I said same parameters,
mum & dad doing fine until this arvo, home from rissole and both dead.
I should mention that they were reintroduced to a tank that contained a
9"Oscar and 4" Jack Dempsey, now call me dumb but these two Convicts have
been in and out of this tank with the same company for 18 mths, also they
were'nt eaten they just curled up together and died.
Both tanks are equipped with u/g, mechanical ie; cannister or in the case
of the 30 gal two moderated hangover the back filters,the 55gal also has a
2000 liter internal filter to accomodate the waste that these suckers
produce.
Any advise would be appreciated as I'm stunned!