Breeder/Fry Box Recommendations . . .

[Wookii]

With the Tetra it is likely that they are spawning if you have a relatively low pH. The eggs are so tiny they are almost impossible to see. However, the eggs won’t develop under your lights. Normally after spawning you need to have quite a few days with lights off (or at least shielded from the light somehow). Also they hatch very quickly (48hrs after spawning) the fry themselves are also almost impossible to see until about 1 week after spawning.

Thanks Steve, I wasn’t aware of the light requirement - maybe why I’ve had no tetra self propagate in any of my tanks.

I use dark green too and a mixture of floating and sinking to go cover all basis.
I've not tried it but apparently 'eyelash yarn' makes picking eggs much easier as you can run your finger down each thread and feel the eggs. Picking rainbow eggs is a real labour of love.
With regards to the corydoras there's one breeding report on planet catfish, looks like they're happy to spawn on flat surfaces.

I’ve ordered a couple of mops - looks like they’re all just made of standard wool?

I have a couple of moss growing plates with Miroshaki growing on it, so I might add those to the sand to see if it induces anything for the Cories.

 

[ScareCrow]

I’ve ordered a couple of mops - looks like they’re all just made of standard wool?

They'll be fine, I use standard (acrylic) wool to make mine. Most advice I've read says to put them in hot water before use to remove any dye that hasn't been fixed. I used boiling water and it made the wool of my first mop curly. It looks more natural but makes it so much more difficult to find eggs so I'd suggest warm water would be sufficient.
I've not used or seen anyone else use eyelash yarn but Mitchell Broome mentions it makes it much easier to find eggs in this video.
For corys I don't use tubs like he does for hatching as it's more hassle and having the eggs in a fry box has the benefit of the additional water volume so no need to change water.
For my pseudomugil luminatus I do use tubs at first as they're so small and struggle against the slightest flow. The movement on the surface of the water when adding fine powdered food is enough to push them around. I imagine threadfins my be the same, though I've not looked into breeding them.