There are a few species of pim cats from SA that look like this. One is the Brachyplatystoma filamentosum. This is a huge fish that reaches lengths of 360cm and weights of about 200kg. Another is the Brachyplatystoma rousseauxii. This is probably the fish you saw. They also get very large but not as mush so. They max out at about 190cm.
There are several pims that are possibilities but as stated by gman most get pretty large. the are also a couple driftwood cats that it might be (genus ageniosus), there is even a grey sorubim. the picture in the middle is a Goslinia platynema.