The amount of time it takes for tap water to be safe for fish tank depends on several factors, such as the type and concentration of contaminants that may be present in the water. Generally speaking, it can take anywhere from a few hours to up to 24 hours or more before tap water is considered safe enough for use in a fish tank. To ensure your tap water is safe, you should always let it sit out and aerate overnight before adding it to your aquarium. Additionally, you may want to consider investing in a dechlorinator or purifying system that will remove any harmful chemicals and metals from the water.