We combined complete-state matching with direct ammonium excursion analysis to assess the effluent consequences of interventions that save aeration energy. Fixed dissolved oxygen (DO) references of 2.00, 1.90 and 1.80 mg O₂/L were compared in Benchmark Simulation Model No. 2 (BSM2) from 42 restored plant states, with identical influent forcing within each 24–48 h comparison. Separate reruns and metric recomputation checked numerical reproducibility. Annual simulations shared a common initial state over days 245–609. At 48 h, median BSM2 aeration-energy indicator (AE) fell by 1.497% and 2.963% at 1.90 and 1.80 mg O₂/L, while flow-weighted effluent ammonium rose by 4.776% and 10.230%. Both responses occurred in all 42 contexts. Annual AE decreased from 4225.433 to 4164.423 and 4104.719 kWh/d, ammonium increased from 0.474291 to 0.501401 and 0.532616 mg N/L, and time above 4 mg N/L rose from 35.75 to 41.50 and 48.75 h. Total nitrogen and the effluent quality index nevertheless improved, and the annual ammonium 95th percentile remained below 4 mg N/L. Under a study-specific post hoc non-aggravation rule, 21 of 168 candidate–horizon comparisons failed for NH₄, with two additional BOD₅ failures (23 overall). Combining matched plant states with direct excursion analysis revealed ammonium penalties that favorable aggregate indicators did not capture. These effects are limited to one BSM2 realization, its influent sequence, the retained PI configuration and the three tested DO references; their magnitudes do not establish electricity savings or ammonium risk at a real treatment plant.