There are tons of good reasons for going RO
(reverse osmosis) water filtration. First, If you are starting with pure water, you will always know what you are putting in your tank. Some of the types of chlorine used by water proccessing plants are not easily removed by evaporation and/or dechlorination chemicals.
If you live in an area like myself, you may have a lot of phosphates in your water and such, which are perfect for growing green slime algae among other types of annoying algae.
It tastes good.... I recently purchased a system from a seller on ebay (Great guy, called me from canada to answer a question I left on his machine) and It came with a splitter and valve that he through in for free. This made it real convenient for me. I just got about 25" of thin water hose, hooked it to the check valve and now when I need to top off my tank I just pull this tiny hose out from under the sink and fill my tank.
When you factor in the fact that 2 nice fish can cost you almost as much as I paid ($105+ shipping) for this RO
(reverse osmosis with Dionization <- Removes minerals as well) unit It is worth the peace of mind to know that nothing will be in the water to hurt them.
Also, its fairly easy, I have never plumbed
and did not have too difficult of a time setting up this whole system, and I am not the most mechanically inclined person on the planet.
All in all, I LOVE my RO
unit and have had several of my friends decide to buy one after seeing mine.
If you have any more questions, including setup if you decide to get one, let me know and I will be more than happy to help you out. Also, I can put you in touch with the guy who sold me mine, Less than half the price I would of paid for the same style unit at Sears.
Hope this helps you decide!