If it is actively leaking, shut the cold supply to the heater now. A leak from the tank itself usually means the tank, and a tank does not get repaired.

The glass lining fails from the inside once the anode rod is spent, and the steel behind it rusts through. Water from a fitting, the relief valve, or the drain cock is a different matter and is usually a part.

Shut the cold inlet valve on top, then find where the water starts. Dripping from the relief valve or a union usually is a part. Water seeping from under the jacket usually is the tank, and that one does not wait.

Do not buy water treatment to save a leaking tank. Once it is through the lining the tank is done, and no filter upstream reverses that.