Aquarium water can be softened in a number of ways. If you've used testing strips to determine that your aquarium water needs softening, remove some of the tank's water and replace it with rainwater then test again, repeating until you have the softness you desire. You can also soften water by using a water softening pillow, which contains materials that filter out minerals. Alternately, add peat moss to your aquarium filter to help filter out the minerals.