I don't see a strong scientific conclusion that pulling round fish out of the gene pool will reduce the incidence of future round fish. Roundness of any particular fish has no relationship to its fitness to survive and reproduction in the wild. Without selective pressure, the allelic frequency for "roundness" should remain relatively constant among every individual in the population. The "best" discus represent segregation of a combination of "roundness" alleles. The remaining gene pool should continue to produce such combinations at similar frequencies.