I have 2 pair of spawning onyx, a pair of spawning skunks, a pair of spawning tomato's,. Reciently gave away a pair of spawning clarkki, and last year gave away a pair of spawning fire clowns.
My onyx have about 50-70% success rate.. My skunks are still young and only about a 10% success, but things will get better. I havent tried to raise the tomatoes, nor the clarki, The fireclowns produced a few babies befor I realized that they should not be allowed to reproduce because they are down right EVIL!
I didnt buy any of them as pairs (with the exception of the fire clowns). I bought them all as young juveniles and let them pair themselves
I truly believe that besides making them comfortable/feel at home, that a good diet plays a role in their spawning habits