A change in mixing intensity, by alternative aeration device or power input, will directly influence the dissolved oxygen (DO) concentration in the water. The DO level is not only essential for the metabolic activity of cells within aerobic flocs but it is also thought to influence floc structure. A trend towards larger and more compact flocs at higher DO concentrations was noted by Wilen and Balmer (1999), although no clear relation could be found with average floc diameter. Poorer settling properties, a sludge volume index (SVI) of on average 250 mL g−1, occurred at low DO values (0.5–2.0 mg L−1) compared to settling at higher DO values (2.0–5.0 mg L−1) where the SVIwas ca. 100mL g−1. This can be ascribed to the presence of a higher amount of filamentous bacteria compared to the zoogloeal bacteria at DO levels of less than or equal to 1.1 mg O2 L−1 as was observed by Martins et al. (2003). As filaments have a higher affinity towards oxygen, they are able to outcompete their zoogloeal counterparts at periods of oxygen limitation and thus dominate the microbial flocs (Martins et al., 2003). From the previous, it can be expected that bioflocs with a higher floc volume index (FVI) are produced at lower DO levels in the bio-flocs ponds. We suggest, although experimental values are lacking, that the FVI should be higher than 200 mL g−1 to avoid the flocs from sedimenting too fast in regions of lower turbulence. This gives the aquaculture organisms enough opportunity to filter the flocs from suspension before they sediment to the bottom of the ponds and are lost as food. Negative impacts of a higher FVI however, like e.g. possible clogging of fish gills, have to be taken into account as well. In addition, the growth characteristics and stress resistance of aquaculture crop species largely depends on the amount of dissolved oxygen available in the water (Colt, 2006; Huntingford et al., 2006). For instance, exposing channel catfish to periodic oxygen levels of less than 1.5mg L−1 results in a decrease of food consumption by the fish, a lower average body weight and a decreased net production (Torrans, 2005).