Hello, very good to see the interest generated in the diversity of Hydrocynus, and this most useful collation of diagnostic information. The diagnostic key illustrated with comparative drawings is from this paper:
Cotterill, F. P. D. & S. A. Goodier (2009) How many tigerfish species? Genetic insights into the evolution of Africa's Tigerfish and the taxonomic status of Tanzanian Hydrocynus. African Fisherman 20 (6): 37-41
The pdf can be downloaded from my website: https://sites.google.com/site/fpdcotterill/research/tigerfish-evolution
Cotterill, F. P. D. & S. A. Goodier (2009) How many tigerfish species? Genetic insights into the evolution of Africa's Tigerfish and the taxonomic status of Tanzanian Hydrocynus. African Fisherman 20 (6): 37-41
The pdf can be downloaded from my website: https://sites.google.com/site/fpdcotterill/research/tigerfish-evolution