Rosetails have a number of issues, though I agree they are pretty. The excess in branching causes spinal problems, bad scaling, shrunken ventrals, short bodies and a much more difficult time carrying their finnage.
I remove eggs so I dont worry about large numbers but for culling after they hatch they go through a period of a week where I reduce feedings by 50-75% depends on the number I want to cull and the weak cant find the food and die off. Its sounds kinda mean but that and getting eaten, something that happens in the wild. Other then that I have used clove oil for older fry.
Maybe one day I'll try breeding some sort of tetra but to me breeding bettas sounds too complicated/difficult. 0_o
I heard alcohol can be used but that could be a myth. Apparently it's meant to stop the fish from feeling pain. This could be true because they used to use alcohol as a pain killer when doing "operations" but I don't know.