Three individual fish per aquarium were euthanized with an overdose of MS-222 (200 mg/L water). The entire intestinal tract was aseptically removed from each fish under a laminar flow hood, transferred to a sterile 1.5 mL Eppendorf tube, and placed immedi-ately in an ultracold freezer at −70 °C. Frozen intestines were sent to Research and Testing Laboratories, LLC (Lubbock, Texas) for DNA extraction and analysis of microbial populations using the 16S rDNA bacterial tag-encoded FLX amplicon pyrosequencing (bTEFAP) tech-nique described by Dowd et al. (2008). Populations of bacterial genera in the LOW and MOD systems were compared using the Shannon–Wiener diversity index (