There are many diseases other than ich. People advocate quarantine for a reason. While it is true that most fishes are able to fight off ich, all you need is to introduce a fish which might have "ich". If it ends up to be marine velvet, there is pretty much nothing you can do by then. Skin lesions are often secondary infections, usually of bacterial origin. If you are able to get hold of a bigger UV, it would help greatly. I use the Coralife 18W for the return to 10 gallons.
The best way to deal with disease without a QT is to get a big UV which can accomodate the flow of your return pump, without restricting the flow. Pond UVs come to mind. Halve the recommended flowrate of the UV unit (usually stated for freshwater) and you'll get the maximum flow you can use for seawater. Plumb the UV to the return before it enters the tank, this will treat the new incoming water. Next time you get new fish, if you do not have a QT tank, give it a 3-5 min freshwater dip (pH adjusted same with your tank). Use your sump as the QT area since whatever parasite which comes with the fish will pass through the UV before it enters your tank.
I have heard of cases where even the most resilient of fishes which have been healthy for years, just die like flies once they encounter marine velvet.