I think I'd stay away from cedars and redwood, since they tend to have natural fungicides and insecticides; who knows what effect they may have on fish in a confined tank. I'd avoid pine and other softwoods; what with pitch and their easy tendancy to rot. But I think most hardwoods would be OK.
I bet those manzinita branches often used for bird perches would be pretty good.
FYI, 'softwoods' do not have more tendency to rot as many think. cedar, pine cypress are all called softwood but cedar, cypress and some species of pines are resistant to 'rot' as a matter of fact you can find cypress dirftwood on line. species of pines(pinacea) readily grow in wet areas and are resistant .
many softwoods are acceptable but may require closer scutiny when submerged in a tank..however, you may find some that has been exposed to waters(river banks) that may already have leeched much of thier resins out.