Inspired by the electrical discharge mechanism of electric fish, we develop an agarose-hydrogel power source that harvests energy from ion concentrations gradients. High- and low-salinity gels are coupled with ion-selective polyelectrolyte membrane gels via Donnan exclusion to generate electricity. Hydrogel composition was optimized to balance power output and mechanical stability. Asymmetric redox couples (ferrocyanide/ferricyanide anode, vanadium oxide cathode) further increased voltage and current. To prevent gradient depletion during long operation, a continuous-flow configuration was introduced to replenish ions. Simulations and experiments show that flow maintains stable ion distributions and enables sustained power generation. This safe, scalable, water-based platform is promising for bio-interfacing and soft energy devices.