How big is the tank? (Plecos should be in tanks 20g and up) And was it properly cycled? (Takes about 3-4 weeks)
You honestly should of done a large water change, or for a tank smaller then 5 gallons (or a tank not cycled fully) then a complete clean out before adding in a new fish.. especially since you put in AQ salt- when not used correctly, the bad bacteria will become resistant towards it, so any beneficial properties it has for clearing out bacteria/infections is pretty much lost.
Also, when adding a new fish into a tank with other fish, best to QT for a week or two at least before introducing the new fish.
I suggest keeping up on your weekly water changes very carefully to try to get the water cleared out.
Breeder so I have too many to count and too many named to list here!