This paper deals with a technical description of an expert system for the freshwater fish-farming industry. The expert system is connected with on-site measurement equipment and by means of expert control rules supervises and controls the fish production process. Additional features such as server-clients communication via the Internet and an ability to extend a rule-base with new expert knowledge makes development of expert systems for any number of fish farms possible. Access via Internet enables better supervision, planning and better utilization of production resources, which results in turn with increased quality of fish, increased fish growth and bigger profits.