The algae doesn't seem too bad but it's important to get it under control before it spreads too much. Since you don't have corals, you can reduce your lighting schedule to 4-5 hours or so but I don't think there is a need to go completely dark. With those fish, I don't think it makes sense to keep adding snails that are just going to be eaten.
Remove as much as you can manually then do large water changes once you've replaced your RODI filters.
How big of a water change do you suggest...the most I can do at once is 55 gallons

