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We carried out enclosure experiments to assess the potential deleterious effects of the alien species common carp (Cyprinus carpio L.), mosquitofish (Gambusia holbrookii Gir.), and pumpkinseed sunfish (Lepomis gibbosus L.) on water quality and food web in a severely degraded floodplain wetland, the Spanish National Park Las Tablas de Daimiel. With addition of either carp or pumpkinseed sunfish, chlorophyll a and turbidity levels, as well as concentrations of total phosphorus and total nitrogen increased. The magnitude of this increase depended on fish species and was severest in the carp treatment. Mosquitofish did not significantly affect water quality. In the treatments with fish, cyclopoid copepods and rotifers dominated, while cladocerans were virtually absent. Zooplankton biomass was significantly lower in the carp treatment compared with the control. However, no direct negative effect (predation) of carp was observed. Zooplankton biomass did not differ from the control, neither with pumpkinseed sunfish treatment nor with mosquitofish treatment. Nonetheless, both fish species had a negative impact on zooplankton biomass owing to planktivorous feeding. Ceriodaphnia reticulata Jurine populations increased in the fishless controls. Only this cladoceran species was able to control significantly phytoplankton. Results indicate that biomanipulations could have a high potential for environmental quality improvements of degraded riverine wetlands. A model was built to predict the potential advances and successes of biomanipulations in the wetland.