water contains all sorts of great minerals, RO water removes all of these minerals so you are left with pure H2O-- no magnesium, no sodium, nothing. This is why I'm against drinking just RO water, because you lose so many minerals. Likewise, it isn't good to have your fish swimming in straight RO water. I use about a 60/40 spit RO/tap in my 45 gallon because my tap water is really hard and basic. However, in my currently empty and cycling brackish tank, I used tap water only, because from what i read, the species like harder water. So I'm not going to treat it.