I used tonic salt when I first started keeping guppies at the rate of 1tbsp per 20L, stopped using it for a while as I didn't really think it was doing much and started getting a simular problem.
Started using tonic salt again and it stopped. Not sure if it was parasites or just that they like having some salt in their water (around 2ppm). Either way my livebearers (guppies, platies, mollies, swordtails) all seem happier with a little tonic salt in their water.