“Cardinal Tetra”
Essentially all the tetras I’ve kept like to spawn in the morning at first lite. Cardinals are the exception: they spawned only in pitch black, in the middle of the night. This simplified things a bit because I could put a pair in a spawning tank, cover it with a box, and then just forget about it for a few days; no need to gradually adjust the lighting as a spawning trigger.
In my notes about neon tetras I said that I wasn’t sure if the eggs were light-sensitive but I kept them in the dark just to be careful. Cardinal tetras I can say are for sure light-sensitive because if I uncovered the box too soon (even after the eggs had hatched but before the fry were eating) a lot of them eggs or fry would drop dead.
With the combination of very-low tds water + total darkness i was able to raise a number of these, although I never got consistent enough results that I could have produced them in bulk.
A couple of years later I discovered that a tetra community tank that contained some of the first batch of cardinals was producing cardinal fry without intervention. So it’s possible to get fry from this species with a hands-off approach, although I think that tank may have been charmed in some way. This was when I was running my waterchange system with pure RO water so the combination of lots of moss + soft water + (extreme luck) did the trick.
I don’t have any photos of those early spawns, but I do have photos of the charmed tank, which in addition to cardinal fry also turned up Hyphessobrycon vilmae and Hyphessobrycon roseus fry: