you could have a really nice colony of African Clawed Frogs - - whether x.laevis, x.borealis or x.tropicalis.
None of these species can be kept with fish!!! Best kept in species only tanks!!!
X.Laevis are the common ACF you find in petstores - most often in the natural or albino variety. In a 42 gallon you could easily have 6-8 depending on the power of your filter and the foot print.
X.Borealis are a smaller ACF that can be obtained through private breeders. A smaller frog that has a grayish body with spotting and a blue hue to it in the light, underside is yellowish with gray/purple spotting. You could have 10-12 in a 42 depending on filtration and foot print.
X.Tropicalis is another smaller ACF that can be obtained through private breeders. A small frog that has a silver/gray body and tiny silver eyes. You could have 10-14 in a 42 depending on filtration and foot print.