Tap water is treated for human consumption and the treating of the water adds elements not suited for aquariums and fish.
This is where reconstitute RO is ideal when using RO (Reverse Osmosis) and Rainwater as a base product and then re-mineralizing creating water best suited to fish and aquariums similar to what is found in their natural habitat.
Reconstitute RO is a high quality, multi-component, broad-spectrum mixture of all mineral ions including General Hardness (GH), alkalinity (KH), and all major, minor, and trace minerals required for optimum health and longevity of freshwater fish, invertebrates and plants.
When mixed with filtered water prepared by reverse osmosis, distillation, or deionization, it will convert those filtered waters into a natural environment in which all aquarium inhabitants will thrive.
It has been used to raise discus and other freshwater fish, as well as keep whole systems in a breeder, public, and home aquariums.