This study focused on the test of two hypotheses that can occur in an aquaponic system. 1. Smaller fishes result in higher fish and plant production due to fast metabolism, and generation of waste nutrient for plants. 2. Carbon addition results in greater fish and plant production, and lower total ammonia nitrogen (TAN) levels than without, similar to what biofloc does in an aquaculture system. Two blocks (I and II) of experimental units, all were consisted of three fish sizes: small (9 g), medium (16 g) and large (41 g), each with two replicates. Rearing experiment was carried out for 49 days. Each experimental unit was a raft-aquaponic system, which accommodated similar biomass of tilapia (Oreochromis sp.), 83 g, and cabbage (Brassica oleraceae var. capitata L. f. alba), 74 g in a 70 L tank. Six plants of cabbage were fixed on styrofoam. Fish were fed a commercial tilapia feed containing 23% crude protein. Daily feed ration was decreased from 6% to 3% body weight over 49-days. Fish were fed evenly at 1000 h and 2000 h. Molasses was added as a carbon source to block II on days 36, 40, 44 and 48, based on nitrogen input, to attain a C/N ratio of 20. No fish or plants died. Small fishes exhibited highest weight gain (YIELD; 151.1 g), YIELD% (180.39 %), specific growth rate (0.92), protein efficiency ratio (2.70) and lowest feed conversion ratio (1.62) in both experimental blocks. Fish size and carbon addition had no effect on plant growth. Block I (no molasses added) exhibited higher final TAN and phosphate (PO43-) levels, but lower biofloc volume and pH. Importantly, carbon addition did not impede the conversion of TAN to NO3-, and water-loss in both blocks averaged 1% per day. Greater feeding efficiency of small fishes may therefore reduce production costs in the early stage. Finally, mechanical and biological filters can be substituted by carbon addition to maintain suitable water quality and produce biofloc, a secondary fish food source.