The farming of abalone, Haliotis midae L., can be intensified in serial‐pass systems, but water re‐use increases the concentration of NH₃ (free ammonia nitrogen, FAN) and reduces water pH. Changing the percentage dietary protein from 33% to 26% reduced the concentration of FAN (F_{42, 252} = 2.79; P < 0.0001) in a serial‐pass system and did not reduce weight gain (F_{1, 12} = 1.09; P = 0.31) or length gain (F_{1, 12} = 1.08; P = 0.31). Low water pH was the most important variable to contribute to a reduction in abalone growth (weight gain: F_{1, 19} = 64.5; P < 0.0001; r² = 0.76; length gain: F_{1, 19} = 41.9; P < 0.0001; r² = 0.67). In addition, supplemental oxygen (103% saturation) improved length gain (t = 3.45, P = 0.026) in abalone exposed to an average FAN concentration of 2.43 ± 1.1 μg L⁻¹) and an average pH value of 7.6 ± 0.13, relative to a treatment with no oxygen supplementation. Thus, in an abalone serial‐use raceway with three passes, FAN was not the first growth‐limiting variable. It is suggested that future studies should examine the major causes of reduced water pH in serial‐use systems and their effect on the growth and health of H. midae.