Inspired by recent reports, many geneticists would agree that it is worth looking into aquaria or even replacing the Drosophila bottles in their laboratories with fish tanks. Indeed, encouraging results from large-scale mutagenesis screens in zebrafish have promoted this organism to a prominent position in the research of vertebrate developmental biology. Some other small aquarium fish species still inhabit side streams or backwaters of science, like the zebrafish did before, serving smaller communities of researchers as tools in fields such as developmental biology and neurobiology. One such type of fish is Xiphophorus, which offers a genetic system for the analysis of molecular mechanisms in tumorigenesis.