Whit spot diease(AKA ICH) is the most common, but not the only diease that will cause your fish to itch. And all it takes is high temp with salt to speed up their life cycle process and dehydrate them to cure, no meds are needed. Velvet,flukes,anchor worms,fish lice, and other worms can also cause the itching and some like flukes, aren't visible to naked eyes. If he scratches more than usual, say at least once in 10 minutes then something is bothering him. most if not all of the dieases other than ich are harder to kill off than ich. But for the same on all fish, I'd start off with .3% salt, raise the temperature upto 86-90F, and do 25% WC every other day(also keep adding salt to maintain the .3% concentration)for a week or two.
If this doesn't work, it's medication time and do NOT use it unless it's the last resort. Anti parasitic meds are really harsh on fish and there's also a rumor that makes the fish sterile since it's so strong. Jungle parasite away is a good anti parasitic meds containing praziquentel, which is a good medication for parasitic worms. Never medicate without knowing "which" parasites you are dealing with. Protozoan meds(Ich,velvet) will NOT kill almost any of the parasites other than those two and won't do anything.
On soft water fish, hard water(high GH) which also means PH is high(not totally but usually yes) will cause them to itch since they feel the hardness and is bothering them. I'd say your check water parameters, ammonia,nitrite,and nitrate levels along with your PH,and GH. This usually isn't the issue for FHs but do check your GH and PH levels.
Hope this helps and your fish feel better.
Happy new years everyone!
JP