Nah Honestly im not Very sure but ill have a stab and say
1. Piranah
2. Wolf Fish
3. Bull Shark (I also knew they were in between salt and fresh)
Oh and yes Dr_Shakalu it would be pretty random to be attacked by a shark in a river havent heard of a person yet been attack but cross our fingers no one does get attacked =(
Number 1 = fahaka puffer, they will tear into any fish in there tank no matter how big or small the other fish is. And even if a big fish was to eat it the fahaka packs the 2nd most deadly poison known to man in its flesh. So even if it gets eaten it still wins. For another fish to beat a fahaka in this they would have to be able to destroy it and live. any takers?