I would err on the side of not adding the sea salt, mostly because I have heard that pleco's don't do as well with salt in general. What you did add is probably fine. Unless you are treating a disease it is fine not to add salt to the tank, it will do quite well without it. The exception would be if you are raising mollies as those do enjoy at least some salt. There are probably other exceptions without getting into brackish conditions, but in general you will be fine with full freshwater. Keeping salt in a freshwater tank can make it harder to use salt as a remedy for diseases as well as the fish will be more used to it.
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