I would move that fish into a QT tank immmediately and do a temp and pH adjusted FW dip for 3-5 min. Add several drops of methylene blue and aerate with an air stone for about 15 min before dipping the fish. For the dip you can do 90% FW and 10% SW. All you need for QT tank is a 10g with a 50-75w heater and a powerhead of aquaclear mini. Then do every other day 25-50% water changes and feed sparingly. If he doesnt perk up after 24hrs then try adding a general antibacterial and paracidal antibiotic.
Dont worry about the FW dip as puffers tolerate this extremely well.
By the way, are you sure the new puffer isnt being harassed by the other puffers? Its been my experience over the years that many puffers do not get along with each other. _________ 75g reef with Nova Extreme 8 bulb HO T-5 lighting, refuge, Remora skimmer, DSB, Seio powerheads, Acro's, LPS corals, assorted zoo's & mushrooms, Yellow Tang, Maroon Clown, Blue Cleaner Goby , Cleaner, Fire and Sexy Shrimp, Coco Worm, T.Crocea Clam, Derasa Clam
120g FOWLR, 260w Orbit power compact lights, Remora Pro skimmer, 30g DIY Sump/Refuge, DSB,Seio Power heads, Powder Blue Tang, Purple Tang, Flag Fin Angel, False Eye Puffer, Copperbanded Butterfly and a Moorish Idol  |