According to North Carolina State University, it's safe to use tap water for many houseplants. For some, water treatment and processing chemicals like chlorine and fluorine can damage the plant. In those instances, you can still use tap water, but you'll need to take extra steps to help dissipate these chemicals.
Determine if your plant will tolerate tap water. This is difficult to determine without some trial and error, but according to North Carolina State University, plants with long, thin leaves and thin, sprawled-out roots typically don't like tap water.
Look for signs and symptoms that your plant is not tolerating tap water well. Look for brown, burned-looking leaf tips. This is the most common indicator that your plant is sensitive to water treatment chemicals.
Avoid using tap water with perlite-rich soils. Perlite is already high in fluoride, and combining fluoride-treated tap water can cause fluoride toxicity. Use a different soil or use distilled or purified water to avoid this condition.
Don't use softened water because it increases the salt content in your soil to dangerous levels. These salts build up in the soil over time and burn the root system. They also cause leaf and stem damage, which first manifests as brown spots on your plant's leaves.
Prepare your tap water. Fill an open container with a wide base with tap water. Use the widest possible base, such as a bowl or bucket, to allow greatest surface contact with the air. Allow the water to sit out for 24 to 48 hours--many of the treatment chemicals will dissipate, making it safer to use on delicate plants.