Because salt comes in different grain sizes, measuring by volume (tsps/tbsps) is not always effective.
If you measure by weight, you will always treat with the proper amount. And salt is salt, NaCl, so if it says NaCl on the package it will work, the term "aquarium salt" is hyperbole.
I use 3 lbs or rock salt, or water softener salt, per 100 gallons of tank, (and sump, or filter water). This weight produces a salinity of 3 ppt (parts per thousand,) any salinity under 3 ppt does not kill emerging ick.
If you do not treat with the proper amount, to coddle your clown loaches, you may be sentencing the entire tank to death.
Sometimes you will need to experience some collateral damage to save the many. I know this sounds harsh, but its the reality.
If one fish is showing signs it means you need to treat them all.
Each ick spot becomes, or morphs (in the parlance of our time) into up to 100 new ick parasites, and its by killing the new ones that you actually combat and stop the disease, because ick are only vulnerable to being killed when in the new born stages. This also means you may need to treat for at least a couple weeks.
You are not killing the ones on the fish, they are immune to any and all treatment, you are trying to kill the emerging invisible ones.
The UV may kill the free floating new ones pulled past the light.
Vacuuming the substrate (tank bottom) to waste, may also help remove emerging ick, but then a proper weight of salt must be added back with the gallons of replacement water.