If it's a chemtec pump on top of the 15 gallon tank, it could be that the ball and spring (check valve) in the middle of the injector are not sealing properly, allowing water to come back through the check valve and filling the tank. Some of the injectors have a spring and some don't. Relieve the pressure in the system and take the injector assembly apart and clean it. Be very careful not to overtighten the plastic fittings, and there is no teflon tape applied to the two halves of the injector, it's the nylon washer(white) that seals it.
Hope this helps.