Because the water is being cycled through the system, there is no need to change the water. Some water may be lost through evaporation. However, if you decide the water level is running low and needs replenishing, proceed with caution. Adding water to the system can throw off the chemical balance of the system, so add small amounts of water frequently to compensate for evaporative loss. Also, because there is chlorine in tap water (used to kill bacteria), you should ideally let the chlorine “gas off” before adding it to your system. This is as simple as filling up a bucket with tap water and letting it sit, without a lid, for 24-48 hours.