A low water level can result in mould growth on the surface of the liquid and on the weights as well. It is important to keep an eye on the water level (at least once a day) and to leave the crock away from direct sunlight, as this depletes the water level in the ‘moat’.
The moat, creates an airlock, which also helps ensure no bacteria enters the crock, while letting gases escape. So ensure this water level is always at or above the top of the lid holes.