Although a sump is always best for a saltwater setup, there are other things you can do for those locations where it is not possible. First is the skimmer. That will skim out the poop and lots of the nitrogenous waste keeping your nitrate levels low. But, if they do start to rise, thee are a couple things you can do to lower nitrates (keep these ideas in your back pocket);
1. Vodka dosing. If your drunk enough you won't care about nitrate levels. No, seriously, a very small amount of methanol (vodka) in the tank daily will help the skimmer remove nitrogenous waste with longer protein strings. I ended up dosing less than 1ml per day to reduce nitrates in my 30g.
2. Nitrate Destroyer. This bottled bacteria eat nitrate. Unfortunately they cannot establish themselves in an oxygen rich tank environment and have to be added daily. I was also using this at about 3ml per day. Also requires a skimmer for this to work.
filter. Running a filter with resins or other nitrate filter packs. Or you can take all the filters out of it and put chaeto in there with a light shining in. I tried this and it worked some, but not great. Depends on the filter as well.
So running a large nano without a sump can be done, but you may have to tweak it a bit to find what works best for what you have available. There are some things that will prefer the dirty water that comes with higher nitrates in a sumpless system (higher being anything over about 5ppm). My large feather duster lived happily even when my trates were up to 80ppm. he died when I moved him to a tank with 0 trates.