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Pseudoplatystoma tigrinum
This should be a marine fish. It looks like a cat fish in the fresh water.
I found this fish at the market.The owner of the market had thrown this fish to the dustbin!........Thank you very much for the ID Ashley!!!
If my ID is correct, this species is native to South America, particularly in the Amazon and Orinoco Basin. I read somewhere a suggestion that it is found in habitats where it is not native. If you found this fish in the wild around Sri Lanka, and it is indeed a Tiger Sorubim, then I would voice some concern.