If there is one pair in a small tank, then one cave is enough. Especially the slim species ornaticauda and parvulus are prepared to move eggs from the cave in which the spawning took place to another, if there is another one.
The cave should be placed in such a way that you could look easily into it with a torch. If you have seen the eggs, the light young larvae, the darkening of the larvae some days later, still hanging on the ceiling of the cave, and finally their dashing around with the male trying to catch them and bring them back, everything still within the cave: then perhaps a week after spawning the larvae will leave the cave and you will not see them, especially if the floor of the tank is covered with leaves of oak or beech.
Only weeks later you will find one, two, three and finally more very small young mostly beneath the surface. But you have to feed whole the time tiny amounts of Rotatoriae, Paramecium and finally freshly hatched (!) very small Artemia naupliae.