The introduction of new species into aquatic communities to increase fish production is a management technique that often has created more problems than it has solved. In large measure, this is due to the inability to correctly predict impact. Loop analysis is a tool of systems analysis and may be useful in predicting systems behavior of an aquatic community once an introduction is made, using the limited information usually available. Case histories of two lake systems examined by loop analysis suggest that nutrient-poor systems are most sensitive and become unstable after exotics become established. Rules developed by entomologists to guide selection of predators for biological control systems can be adapted for fisheries management. The \^a\texteuro\oeideal\^a\texteuro candidate for aquatic introduction is coadapted with some members of the new system, has a narrow niche breadth, is easily controlled if it escapes from the system, and is free of exotic diseases and parasites.