Live foods may be one of the major factors behind the tetras' willingness to spawn. Peat moss and possibly Sphagnum moss are responsible for it in this tank. It also protects the younger fry from predation. I can not discount the water softness as a yet another factor in egg survival and fertility. It's pretty easy once you learn what to look for.
Females have a paler red colour and the black margin is very narrow and faint.
A bit of foresight in setting up the tank had resulted in the environment where the fry were able to hide untill they got too big to just get eaten. As long as you feed them well with high quality food and the water is acceptable, the fish will spawn. Tetras need specific water conditions and specific set-ups that minimize egg and fry predation. Without these factors the breeding of tetras is difficult. A lot of tetras are still caught in the Amazon delta and imported.