It is always best to follow the medication manufacturer's advice. If theirs can be used in low salinity, then they should know. It's not uncommon for medications to have an abnormal affect when the water is at abnormal conditions.
Your specific gravity of 1.018 is clear now. Did your LFS give a reason for that advice?
You've never vacuumed before? This suggestion was given some time back, I thought. It's usually a good idea to vacuum right after you don't see the white spots. Then you know the trophonts dropped off. Vacuuming then might pick up trophonts entering their next stage or even released tomites. But at this point in time, from your description of the infestation, I would wonder if it would make any difference.
Your specific gravity of 1.018 is clear now. Did your LFS give a reason for that advice?
You've never vacuumed before? This suggestion was given some time back, I thought. It's usually a good idea to vacuum right after you don't see the white spots. Then you know the trophonts dropped off. Vacuuming then might pick up trophonts entering their next stage or even released tomites. But at this point in time, from your description of the infestation, I would wonder if it would make any difference.