More information, please. How much are you going to add, what kind of plecos, is this short term or long term, and what are you hoping to accomplish by adding salt?
They came down with ick so i was going to add some for help with treatment. If i'm remembering correctly i do 1 tablespoon per 5 gallons? The plecos i have in the tank are
L239-Blue Panaque
L201-Columbian Snowball or Orinoco Angel Pleco
L168-Butterfly Pleco
That level of salt should be no problem at all for them. Couple it with increased temps (86-88) and increased aeration.
I had to treat a bunch of wild plecos I collected in Uruguay for ich. In addition to the heat, I treated everything with 1-2 tsp per gallon of salt. No pleco losses.