You'll need both. The RO
filter takes all of the elements out of the water such as, metals,minerals and chemicals.
Here is the RO
filter that I just picked up on eBay.
I see that a lot of people on this board use them and like them. I have not found a cheaper place for a filter yet.
I too am dealing with cyano
and diatom , because I am not using ro
water, plus my tank is still cycling. You will need the protein skimmer even if it is just live rock. I was stupid and bought a cheapo' skimmer from petsmart and it does not work that great so make sure you get a good one.
I am still a noob so I can't answer the other questions about the snails sorry..