Shinchan's super-secret formula for Multie breeding success:
1. Play sand substrate. Fry are almost the exact same color as play sand.
2. However many shells you think you need, multiply it by 3
3. Put some rock in there too, and some plants if the mood strikes you
4. Totally abuse/ignore the tank. Only do water changes every 3 months. Let it become overgrown with hair algae. Never test water params (like ever). Let the internal filter become totally clogged with algae so that there is barely any flow. Allow your 4 year-old daughter to scare the beejezus out of the fish on a regular basis, yell and stomp a lot in the vicinity of the tank, and wildly overfeed the fish. Oh yeah, and make sure you don't dip any plants that you introduce to the tank so that it can become overrun with pond snails (pond snails are like a pet hamster to the multies, they love taking them out to play).
5. ???
6. Profit!