Called my go-to store. They have 2 tanks with juvenile D. filamentosus and one single adult D. maculatus male. I'm really tempted to take the D. maculatus, because the specimen is there for at least 4 months now. But D. maculatus neither fit into the Rio Negro setting I'm aiming at, nor is it a real blackwater species, but rather a clearwater fish. The readings in my tank are already below clearwater levels.