unidentified hybrid tetras

[Harlock]

Genetic diversity is best achieved with new blood (genes), by diversifying the bloodlines with fish of the same species who have either never bred together, separated by several generations with new blood added and by carefull breeding practices and culling of fry/juvies with growth problems and defects.

Obviously, but keep in mind in the wild genetic diversity can come by hybridization as well. I'm not a complete advocate of it, but in the proper hands (like say, a responsible hobbyist) hybridization can be fine.