Tandanus tropicanus is distinguished from its congener T. tandanus from different drainages. In the Murray-Darling drainage, it differs from T. tandanus by having the following characters: 150-165, mode 156 rays in the continuous caudodorsal and anal fins (vs. 136-149, mode 143); 31-35, mode 31, gill rakers on the first arch (vs. 24-30, mode 28); interorbital width 32.9-38.4% of HL mean 36.2% (vs. 40.3-47.7%, mean 43.5%); pectoral spine 11.1-14.0% of SL, mean 12.4% (vs. 8.9-11.2%, mean 10.1%). For individuals >20.5 cm SL, T. tropicanus can be diagnosed from T. tandanus of the Murray-Darling drainage by having 12-19 (mode 14) posterior serrae on the pectoral-fin spine (vs. 7-11, mode 9). Also, individuals >7.0 cm HL, T. tropicanus differs from T. tandanus of the Murray-Darling drainage by having the following characters: length of the inner mandibular barbel 32.3-41.0% of HL, mean 36.1% (vs. 40.9-54.7%, mean 46.7%); distance between bases of outer mandibular barbels 27.4-33.8% of HL, mean 29.6% (vs. 21.2-25.9%, mean 24.4%); distance between bases of inner mandibular barbels 13.5-15.9% of HL, mean 14.8% (vs. 10.4-13.2%, mean 12.0%); distance between bases of maxillary barbels 47.0-53.5% of HL, mean 50.1% (vs. 39.5-46.6%, mean 44.2% ); maximum cleithral width 75.6-81.4% of HL, mean 78.2% (vs. 68.6-75.2%, mean 71.9%); and eye diameter 14.7-17.2% of HL, mean 15.6% (vs. 12.3-14.6%, mean 13.6%).

In the eastern coastal drainages of Australia (Burnett, Tweed, Brisbane, Fitzroy, Clarence, Mary, and Richmond), Tandanus tropicanus differs from T. tandanus by having the following features: 150-165 rays (mode 156) in the continuous caudodorsal and anal fins (vs. 132-147, mode 145); gill rakers on the first arch 31-35, mode 31 (vs. 27-30, mode 28); interorbital width 32.9-38.4% of HL, mean 36.2% (vs. 40.2-47.3%, mean 43.2%); and to a lesser degree, a longer pectoral spine (11.1-14.0% of SL, mean 12.4% vs. 10.0-11.1%, mean 10.5%, respectively). For individuals >20.5 cm SL, T. tropicanus differs from T. tandanus by having more posterior serrae on the pectoral-fin spine (range 12-19, mode 14 vs. 7-11, mode 10).

Tandanus tropicanus can be differentiated from Bellinger catfish by having a larger eye diameter (14.7-18.5% of HL, mean 16.3% vs. 13.2-14.4%, mean 13.8%, respectively), a longer pectoral spine (11.1-14.0% of SL, mean 12.4% vs. 9.8-10.7%, mean 10.4%, respectively) and, to a lesser degree, fewer gill rakers on the first arch (range 31-35, mode 31 vs. 35-39, respectively). It is distinct from T. bostocki by a shorter post-orbital distance (37.4-42.7% of HL, mean 39.5% vs. 46.7-55.9%, mean 51.9%, respectively), a longer snout (44.1-52.3% of HL, mean 48.9% vs. 37.6-42.1%, mean 39.8%, respectively), and, to a lesser degree, a larger eye diameter (14.7-18.5% of HL, mean 16.3% vs. 11.9-15.5%, mean 13.3%, respectively), more rays in the continuous caudodorsal and anal fins (range 150-165, mode 156 vs. 139-149, mode 147, respectively), more gill rakers on the first arch (range 31-35, mode 31 vs. 18-22, mode 21, respectively), and prominent (vs. reduced) posterior serrae on the pectoral-fin spine, respectively (Ref. 96080).