3 pounds per 100 gallons? that seems like a lot but I have little experience in that area
3 lbs per 100 gallons produces a salinity of only 3 ppt (parts per thousand), this salinity will kill emerging ick.
The ocean averages 35 ppt.
2.5 ppt is not enough, and ick laugh it off and go on their merry way, reinfecting
It takes at least the 3 ppt of osmotic pressure to kill young emerging ick.
I use salt by weight, because different grain sizes of salt produce differing salinity if measured by spoon fulls or some other volumetric way, a lessor salinity wimps out, allowing ick to reinfect.