hi again,
don't forget that tap water contains not only phosphates and silicates. There are a lot of bad "stuff" in there. Like nitrates, heavy metals, etc. As well tap water is very hard. probably harder than it should be in the tank. Of course, tap water parameters are vary from city to city, state to state.
If i was you, i would considering to buy RO unit, maybe small one. Like i have Kent Marine's bare Bones 20. It's 20 GPD unit.