yes, freshwater dips stress the coral, it blows their osmosis equalibrium, meaning the salt level in their body is higher in them than the surrounding new water and may cause osmotic shock, this inflush of freshwater tthrough the tissue quickly kills the flatworms but stresses the coral as well, and if left in fresh water too long may kill the coral as well. Ive done the dips and some people like the treatment and some dont but I dont do it for long and have the dip water at tank temp.