Water chemistry of fishponds can be strongly subjected to direct and/or indirect effects of fishery farming and agricultural practices within a catchment. Long-term data on water chemistry of the fishponds in the Trebon Basin are related to changes in the fishery and agricultural practices, which provides an interesting tool for the description of important causal relationships. Results of the surveys in the periods 1954-1955, 1990-1991, and 2000-2001 show changes in fishpond water chemistry and trophic level. The decrease in total dissolved solids during the Last ten years in fishponds reflects changes in agricultural practices within the catchment. In spite of this trend, the concentrations of nitrogen, phosphorus and chlorophyll-a remain high and they are more dependent on fishery management than on agricultural runoff.