Tap water is quite safe for humans, but it's not entirely true for your fish. As an aquarium owner, you shouldn't ignore our guide to making tap water fish safe. To treat harmful substances in water, such as chlorine, heavy metals, etc., I recommend using reverse osmosis. In addition to these substances, it is very likely that there are other harmful substances found in the water. Applying a balm is a necessity to get rid of this unwanted toxic substance.