What are you using for treatment? If it is malachite green then remove the carbon. I always just used a salt bath which you do by raising the temperature to about 80ºF and treating with salt. The high temperature speeds up the life cycle of the parasite that we see as white spot. The parasite is only vulnerable to treatment after it is free swimming in the water, not when attached to a fish, so you want to get it exposed to the salt or other treatment ASAP. I always used about a tablespoon of salt per gallon of water as the treatment and a week or so at elevated temperatures was enough to kill off the parasites. After that start to remove the salt by doing a 50% water change every couple of days to bring the water back to untreated conditions.