You said "darker" which means it's a possibility that you got an Asian Featherfin aka Bronze Knife aka Black Asian Knife. They can average 10"-12"and are one of the most aggressive of the Chitala species. As long as it doesn't have a dorsal fin, it's an African Brown Knife, but also look into a Brown Ghost Knife.they don't have dorsal finds as well and stay relatively small.
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