Even if you have seeded this new tank with some media from your other tank, I would be very surprised for the 125l and its filter to be fully fishless cycled in the space of a few days!
What did you use to test for ammonia/nitrite/nitrate? Test strips or (well shaken) liquid test kits?
What ammonia concentration were you trying to maintain?
Cardinal Tetras could live in typical ~80cm long 125l for life (unlike the Mollies), but hard water is not ideal and I would definitely recommend a prolonged "slow and steady" approach to adding anything else after these initial Mollies, as in give it 3 or more weeks... I 'm expecting you to find you are effectively still "fish in cycling" which means you need to be ready to do upto ~75% water changes daily and adding new fish is going to put further strain on the immature AOB and NOB bacteria colonies.