Testing is always good advice, but I've not heard many horror stories about parasite meds, it's usually the anti-bacterials that cause trouble.
Precisely, any organic material. If I understand it correctly,most anti-parasite meds work by attacking organic compounds in the tank. Ideally the compound you want them to attack is your ich, but if there are a lot of other organics available then they will deplete themselves quicker and less effectively. The irritation caused by meds is due to that very effect. they technically also attack your fish. The fish just stand a better chance of surviving the attack than does ich.
Are you using half dosages ? This is usually not highly effective for ich, and most smooth skinned or delicate fish don't withstand meds very well. Day 6 would indicate to me that you are seeing round two of ich, but a lot depends on tank temperature, and how easy it is to observe your fish. Fast moving fish are hard to observe.
My primary reason for preffering salt is that ICH meds cannot be used on delicate fish most of the time and salt can. IF your loaches are handling a full dose well enough then I would not switch out treatment. if you are not using full dosages or your loaches are showing signs of severe stress then I would switch out meds to salt personally. if you do decide to switch out methods be sure to do it without leaving the tank un-medicated. in other words bring the salt level up, then reduce the medication level via AC or water changes, don't remove meds and then add salt later. As I rule I do not reccomend switching once treatment is started, but if you are not or cannot use full dosages for a full term, the chance of ich making it through is a lot higher. Remember it can hide very very well on your fish, so it is not uncommon for it to fool you into thinking your are done, and then showing up later on when something else stresses the fish.
Dave
