I think small, frequent water changes are ideal. Your nitrate is 20, which is too high, but it's not astronomical. Eventually your nitrate will drop as the nitrobacillus (bacteria that convert nitrate to gaseous nitrogen) colonize the anaerobic layers of your sand bed. FYI, deep sand beds can be problematic down the road. I personally used to keep dsbs, but have since gone to shallow sand beds and rely on waste export rather than denitrification. That's a whole other topic, more appropriate for the General Reef Discussion Forum and not a concern for the time being. Anyway, for now, if you want to increase to daily 1 gallon water changes that would be fine. Or, as Minh said, larger wcs will bring nitrate down faster, but perhaps be more disruptive, and unnecessarily so, in my opinion.