| This is another tricky question because there is quite a bit of debate about using salt in a freshwater aquarium for a lot of reasons...one of them being to prevent illnesses...however, from what I have read, it seems like everyone does tend to agree that it is ok to use salt for a short period of time for treating Ich and similar issues.
The first thing I would say is make sure you have a reason for treating your tank with salt. Like I said, some people use it for preventing problems, which I agree, it can have some effect in doing. There are a few other reasons, such as to help your fish deal with nitrites (another commonly agreed upon temporary use from what I have read on forums, which has been a lot if not too much). Generally it is also accepted that salt is good for livebearers, however your tank is going to be an exception because it is not comprised solely of those sorts of fish....but lets get to the nitty gritty:
Salt is used primarily for the treatment of Ich, and possibly for oodinium (aka velvet) since it is pretty leathal to the ich protozoa when it is in its free-swimming stage of the lifecycle it goes through...meaning not the stage in which it is a white spot on your fish(es). It is important to note though, that this treatment is usually coupled with an increased tank temperature in the range of 85 to 90 degrees, depending on the fish you have and how slowly you can turn that temp up without the fish suffering. The additional temperature of the water will create a more ideal environment for the Ich, which I know sounds contradictory, but the idea is to speed up the lifecycle of Ich, particularly the white spot version, so it drops off sooner rather than later. Once it drops off, it reproduces and those new protozoa become free swimming at which time the salt helps reduce them or eliminate them.
The usual advice is to dose about one tablespoon (three teaspoons) per every five gallons of actual water in the tank...keeping in mind that you tank does not hold its advertized gallonage becasue the substrate, decor, equipment, and even the fish displace some of that water. HOWEVER, when there are scaleless fish, or most tetras, present in the tank, then you would want to cut that dose in half, or one tablespoon per ten gallons of actual water. This is becasue it would irritate a scaleless fish (or tetras...in general anyways) more so than it would a scaled fish. It is also important to dissolve the salt prior to putting it in the tank and then add it slowly, over the course of a few hours, to further protect against irritation of your fish (any type of fish so this is something you should always do).
The salt that you want to be using is regular sodium chloride (e.g. table salt, aquarium...not marine...salt, kosher salt, etc...). If used for a short period of time, based upon my research, table salt with iodine in it is ok since the iodine is in such small trace amounts that it wont cause a problem...in fact, your fish could benifit from iodine to some extent and most inverts such as those who molt like shrimp do, really need iodine to help that molting process go smoothly.
Again, I am sure that you may hear the exact opposite advice from another person...and I can see those points too so it can be argued that in reality, this is an opinion based decision and not a true guideline. Just make sure that you do not overdose the tank by adding that amount every day since you would just be adding it on top of what you put in prior. Also, when water evaporates, it leaves the salt behind so to get it out, you would most likely need to do a water change and at that time, you would want to add more salt in proper dosages.
Hope this helps a little bit. There is quite a bit more to discuss as far as salt in an freshwater aquarium goes, but I think I covered the gist of things as they pertain to using salt for Ich treatment. | 
