The easiest way to do the salt is to get a 1 gallon jug, like the kind spring water comes in. Fill it with dechlorinated water and add the amount of salt you need. When you do the water changes, take out as much water as you can without disrupting your cycle or, if you can, use a hospital tank. Keep track of the amount of water you take out and then pour in the amount of water from the jug. If you need more water, refill the jug with dechlorinated water and salt. This way, when you premake the water, you know that you will always have the required amount of salt. I hope that makes sense. I think I confused myself.
Hopefully this helps Xavier. If he spends some time in the salt mixture with no change, it may be something else and not external parasites. Then we'll need to try other alternatives for treatment.
Good luck and keep us posted! :)