id say your tank or where your source water comes from has ich. ie there could be a lifestage of ich in your source water.
the change in water could do two things, promote the ich and lesser then strenght of the fish against it. often ich will show up after water quality changes, and not neccesarily just from good to bad water. say a temperature drop for example will often bring it on. especially more so if a fish is transported and goes through changes and is placed in a tank that does not neccesarily have ich present ON the occupying fish at the time. it can then somtimes show up only in the fish that went through the changes in water quality and are stressed.
it sounds to me like if you are using salt and the ich dont show then the salt is stopping it. you change water, then bringing in ich and lowering salinity and so it gets a chance at thriving again. you can use salt at 1ppt without much drama.
it is said that salt at levels between 1-3ppt will promote slime production and sloughing off and settle the fish down even. if the pathogen reacts badly to salt and the fish does not then by all means use salt. i have and it works well for me, it is the only thing i would use in fact, but you need to know the life cycle of ich to treat it right. no point in dosing it with salt and then changing the water back to fresh a few days later see or it wont be fully elimanted from your water, fish, gravel.
having said that do not use it if you dont know the rate of dose. 1 ppt is 1 part per thousand parts. or about 1 gram per litre, and research the tolerances of your fish too.
i would suggest dosing it with salt and when you change water, dose the new water too back to the right level. a hydrometer is safest if your dealing with expensive fish. then do that for three weeks and your ich should be gone depending on temps. then if your source water is town supply and treated with chlorine or something like that then you can go back to using that water after dechlorination process ( in the sun or bubbling in a tub for a day) without salt in the future. i wouldnt be running treated town water direct into an arow tank though so thats why i wonder if your source has fish in it to begin with that may harbour the ich.