When the water is paused (no water flowing out of the filter), there should be 5 cm (2 inches) of water above the sand. This allows for oxygen in the air to permeate the biological layer. If there is less than 5 cm, add a little water to compensate for evaporation. After adding the water, measure it again. If there is less than 5 cm, remove a little sand. If there is more than 5 cm a small amount of “approved” sand should be added. Use only “approved” sand – i.e. sand that complies with the HydrAidTM Filter Media Standards included in this Handbook.