Bret, do they sell just RO water on its own???
Cos when you do top offs, you need to use just RO, cos the salt stays in the water, cos it doesnt evaporate...so if you added premixed, then your salt levels would go up and up...
If they dont, then youll need to get an RO unit...
But you will also need premixed saltwater too, for waterchanges.
Also, when storing any of your water, id put a lid on, or some cling film over the top of the container, so you dont loose any water through evaporation
Mikey
EDIT: And as far as fish go, id go for a yellow coral goby. Theyre small and inexpensive, and they look great.