I havn't been able to pick up a RO
unit, but many of the "super" grocery stores have ro
water dispensers which is what I'll be using until I can get one myself. That's a good option with a small tank. You don't need a 'fuge, but as you can see in the picture I have chaeto and caulerpa in my display, I just have to trim it every couple weeks. In such a small tank its not hard to take the rocks out and give them a bit of a scrub down and I think it gives it a nice natural look. You could also make a small one out of a HOB
filter if you wanted.