Sorry to hear about your wels cat. Yarelli are not aggressive, but they do have very sharp backward facing teeth, used to grab food, so they could easily inflict deadly wounds.
as for the piraiba, which is the filamentosum, the true ones are usually found only in Brazil, that is why you don't see them much, its like the peacock bass, considered as a commercially valuable fish, I don't think they can export them. Sometimes you can find one or small group from neighboring countries, also in Suriname, they've caught them before, but very large ones.
