Hi all,
I haven't kept these, but I've given away a lot of moss to people trying to breed them. The feedback I've had is that they are quite easy in an established tank with lots of moss, not just little patches of it but huge cushions and tangles of moss. I don't think it matters if it is bare bottom or has a substrate, because the moss should cover everything. You could use a layer of glass marbles on the bottom, but if you have enough moss you don't need to. I wouldn't use a spawning mop, purely because the eggs aren't sticky.
I would plant up some wood/mesh/aquarium sponge with lots of moss, put it in the spawning tank (probably go for at least a 30cm cube) and leave it to get on with it until it has grown. A larger tank will allow you to grew up the fry to a reasonable size without having to worry too much about water quality. If you could get some Najas or Ceratophyllum for the tank even better, I'd probably add some Limnobium as well. I'd want at least 4 weeks growth.
I would grow some mosquito larvae for conditioning the adults. I culture my own mosquito larvae in the summer. I use a black 5 Gallon "builders bucket" filled with rainwater with a handful of dry leaves and a handful of glass clippings added. I put it somewhere shady and float a cork in it (the mosquito female needs somewhere to perch why she lays her eggs). If you want to contain the grass clippings, you can tie them up in a stocking. If the water begins to clear and mosquito larvae production drops off, add some more grass clippings. This works really well.
I'd then add your pair/harem of CPD to the tank, condition them with lots of mosquito larvae until they are really fat and well coloured, as soon as you see them showing signs of spawning take the adults out the next morning or the morning after (they will spawn evening or early morning). The other requirement is high quality water, so at least 20% water change every day (ideally with re-mineralized R.O. or rain water) and a matured sponge filter (air powered is fine). You won't see the eggs or fry for a while, but after a couple of weeks you should be able to see them darting around the moss.
I've bred Rams, Killis, Tetras, Pencils etc using this approach and it works pretty well for nearly all "difficult" fish, the moss provides both shelter and food, once the fry are amongst it it doesn't matter how small they are they are surrounded by potential food sources in the biofilm.
cheers Darrel