Your new fish may not be sick when you bring them home, but they certainly will be when you hit them with ANY meds. Fish meds are a lot like chemotherapy where the idea is to manage killing the disease without killing the fish.
Depending on the size of fish you are placing in QT, even a 10-gal may not be large enough. I struggled with killing fish in my 10-gal QT and finally gave up when I had a 6" Niger Trigger go from perfectly healthy to hiding behind the heater and starved himself to death. If you can muster the space, I would get a 20-gal for saltwater QT. Keep the 5-gal for a dip tank.
Be sure the new QT is placed away from bright light, frequent people traffic and temperature swings. Definitely cover/paint the back and sides, natural room light works great, supplemental aeration and a HOB filter that allows you to easily change media/foam/poly.
Saltwater fish in QT receiving meds are stressed, oxygen deprived, sensitive to light and noise, may have poor appetites, a long list of challenges until they get past the meds and back to regular clean water. Think of your saltwater QT as a human Intensive Care room where cleanliness, oxygen, rest and proper nutrition are paramount to recovery.