i would treat the whole tank. it also is good practice to "worm" your fish every so often, especially when you get new ones(dont know what conditions they are coming from). does his anus look inflamed or swollen? keep an eye out for his poo. may be best to isolate him until you see what his poo looks like. anything white, stringy,mucousy and it would almost certainly be parasites. those meds work well if they eat the stuff. soaking in garlic or rolling in some bloodworm juice may be advised if the fish find it unpalatable. also the pellets are usually a little too big for bettas (and those loaches), i just crush them up a bit before feeding.
good luck and keep us posted.
cheers
good luck and keep us posted.
cheers