I can see several things not quit right with your set up.
You must use 100% rain water, the calcium and magnesium levels in your water must be next to - if not zero.
The tank needs to be completely covered(blacked out) for 2 reasons,
1, cardinals can be flighty and need the darkness to settle down.
2, the eggs which should be laid on the second or third morning are light sensitive, if the tank is not very dark all the time, the eggs will die.
The hight above the fish appears to be to deep, they only need it to be shallow around 150-180mm deep.
The author of that article used a very basic set up,
tanks were 200x600x200mm.
100% rain water similar to yours
Boiled garden peat on the bottom. To help condition the water, calm the adults and to trap the eggs.
Spawning mops or spawning moss were the spawning sites(plants were/are not required).
Once the adults were place in the tank it was completely blacked out until they had spawned (48-72 hours), the adults were removed tank left covered until fry were free swimming(as per article).