b-well :

Does your overflow pipe drain water from the pond surface or from more than 3 ft. below the surface ? If from the surface you may have lost a lot of your fertile water. One of our ponds looks just like what you described under the same circumstances. If your visa. stays at 20 in. for a week then you may want to add a little fert. but go light and slow to see how the pond reacts. A slight green color and 18-24in. visa. is the general recommendation . ewest