Hi,
I am also new to the world of saltwater. But I do know about the advantages of R/O (reverse osmosis) water and DI (deionized). A reverse osmosis filter removes around 95% of microorganisms, pathogens, chemicals, and other harmful ingredients found in most tap water especially If you live in a major city that has a water treatment facility chlorine and chloromine are almost guaranteed to be found in your tap water both of which are devastating to the chemistry of your water, your fish, and even worse your biological filtration. A good R/O filter will take care of these problems. Look to spend around $150 to $200 for a good R/O system. Or your LFS probably sells it mine sells fresh R/O water for $.75 a gal and premixed salt water for $1.00 a gal.
DI water is also good for your aquarium I am not 100% sure how it works, but it removes ions from your water, it is another way of purifying water. An Ion is an atom of an element that has taken on a charge either positive or negative by releasing or gaining an electron. If the atom releases an electron it will take on a positive charge because there are more protons in the nucleus, and visa versa if the atom gains an electron it will then have a negative charge because it has more electrons then protons. If the atom never releases or gains a electron it is considered neutral and more stable than a charged ion. DI filtration removes these charged ions producing a more stable environment for our fish. Another good reason to use DI water is that it is less conductive of electricity. Because electricity is nothing more than free flowing electrons It will be harder for the electricity to travel through the water because there are no positively charged ions for it to be attracted to.
Good luck from your fellow new aquarist
I hope this helps you understand water filtration a little more.
~Jon
