Aphyocharax rathbuni are one I'd like to do too. For some reason, they aren't common in my local stores.
They take advance prep. Getting young to hatch is one thing. Raising them is another.
Good news number one is that they don't seem to be light sensitive to the degree some other Characins are. So I would start with:
1) 2 aquariums with sponge filters, cycled, of at least 10 gallons each, filled with soft water at around 25c. If one is a 20 gallon, you might be ahead. More on why later...
2) A good supply of live food for adults. Brine shrimp, mosquito larvae, bloodworms, fruit flies, grindal or black worms will do. Frozen will work, but live is better. Prepared foods show their limitations with breeding attempts. They just aren't up to scratch, whatever the brand name or its claims. You can have some success with soldier fly larvae based foods, but generally not as you would with insects themselves.
3) A good supply of infusoria for the young, ready a day or two in advance of hatching. This is where the breakdown strikes for most of us. We prepare fry food too late. Microworms can be stage two, then freshly hatched artemia as they grow. Some powdered foods are okay. I would use hikari's first bites, and krill fines do a good job with other species. I mix them with water in a vial, shake them and dole them out.
4) A good supply of fine leaved plants - hornwort, cabomba maybe guppy grass and java moss. Don't go buy them the week before, because if they don't take and start decaying, you have losses coming. Get them established. A rathbuni eggs are adhesive, and they need plants to stick to. Lots of plants are good, as there should be lots of eggs. But no snails or hydra!
So now you're set up. Split the sexes. Males in the 10, females in the bigger tank where the plants will be. Feed them well for one to two weeks. Next, even if the eggs and fry aren't photophobic as cardinals are, turn off the lights and combine the sexes. They don't need light except to find eggs to eat.
Now I go to Seriously Fish, who say eggs hatch in 2-3 days, and larvae freeswim 4-5 days later. That seems slow to me, but rathbuni are one I haven't tried. I judge the spawning by studying the bellies of the females closely. This is a problem, as unless you see them spawning, it's hard to be sure. Tetras don't always spawn automatically when combined. I've misjudged with some tetras, and either caught the adults too quickly, or left them in so they could eat all the eggs. You have to get the adults out.
Once the tank is sitting quietly, apparently empty, I use an intense led light. I have some that you wear on your head, with straps, but a flashlight will do. If you rattle the tank a bit you can see larvae move on the glass before they are freeswimming, and you know you're in business. They are tiny. NO bright overhead tank lights yet, just in case.
Now, why a 15 or 20 gallon? It's an ideal world option, because several hundred fry can appear. It does mean if you get 500, you raise 500. You will probably lose a lot, so starting with good numbers helps as you learn. You can start that tank shallow so they can get to food, but as the group grows, you need clean water. Lots of water changes are needed. If I can, I do small daily ones, but as they grow, I raise the water level. My last tetra success was the fairly easy black neon, and I raised around 50 starting in a 5 gallon. That was sufficient for me and for my club, as well as for a local store to take 20. If I had used a larger tank, I'll wager I could have doubled that. I lost a few when I had to transfer the gaggle to a larger tank to grow healthy specimens. I wish I'd started bigger.
I have decided to retire 5s for tetras. Too much work. I'll now start with 10s.
And there is a recipe.
