From Seachem's Website
"Keep in mind that this product is still effective as low as .20 ppm and generally safe for fish up to .80 ppm, but we recommend .50 ppm as the ideal level."
The initial dose I get it close to 0.2 ppm, then add again what I dosed to get the initial 0.2 ppm for about 0.4 ppm. No issues so far and it has worked to cure both ich and velvet.