Tap water is often not suitable for fish, is too hard or contains too many undesirable substances, such as silicic acid or nitrates. An osmosis system is a simple and inexpensive way of producing the purest water, which is the starting point for aquarium water. According to the principle of reverse osmosis, the tap water is pressed through a special membrane at normal tap pressure or using a booster pump. Only water molecules can pass through the membrane. Other substances, such as hardness constituents in the water, heavy metals, pesticides, bacteria or viruses are effectively filtered out.