Just a stab in the dark but could it just be stress related to so much activity in the big tank? Maybe not even have anything to do with water chemistry or quality.
Stress induced prior to even seeing movement before being moved out of the 750? Once movement was confirmed they were immediately moved to their own 180 on that system. A 6x2 for a pair of 10" and 11" female Marbles- nothing else in the tank. Not sure where the stress would come from? I'd assume if there was an issue prior to seeing movement then once moved wouldn't the pups' movement stop if there was issues? Both pups were flawless fat lil buggers with no trace of sacks...... The one that was curled makes me think it was born alive, but then died before I got home.....