First off, let me just point out that fish dont "get/make" ich from stress. what happens is that when they are stressed, woudned, or malnurished, their natural immune system fails, and they are no longer able to fight infections properly. Ich is a free swimming organism that can be introduced into your tank from a variety of sources. It hooks onto your fish on the body, fins, and in extreme cases, even around or on the eyes. Because it is free swimming, and reproduces rapidly, it can form swarms in your tank, virtually invisible to the naked eye. I agree wholly with guppy that Ich can hang out in your tank, showing mild cases for quite some time. However, if you notice any spots, no matter where they are on the body of your fish, you should treat immediatly. Ich is one of the most common salt water "bad guys", but doesn't have to be devistating. Do note though, that if left untreated, it can, and most likely will take over all of your fish, ending in a massive funeral for you!

There are several ways to treat Ich. Some over the counter remedies are good, and others do nothing but drop your water qualitly, and have pretty colors. Also, temperature, and salinity levels play a role in recovery. You may also want to try a "fresh-water bath". To do this, you will need a seperate container, I used five gallon utility buckets. Make sure you have 2 parts fresh-water, and 1 part salt water. Not all fish can handle this, so do some research on your particular species before this. You should leave your fish in this mixture for no more than 5-7 minutes. Remove them immediatly if they become severly stressed, or breathing becomes excessively erratic. Most fish will lie on their sides, appear sluggish, and sometimes, not even move. This is ok, and normal. What this fresh-water bath does, is force the organisms (ich), to absorb the freshwater through their soft bodies, litterally making them explode, as they cannot survive with those parameters. This may be uncomfortable for your fish, but not detrimental, as long as it is monitored. Also note, that some over the counter remedies for Ich, can contain copper traces, which are fatal to most all inverts, and can cause serious harm to certain species of fish. Research is the key. There is no 100% way to diagnose Ich with a naked eye, as other infections can appear similar, such as velvet disease. If you can, the best suggestion I can offer, is treat early, and if possible, take your fish to your local vet's office, so that a scale scraping can be done. This way they may look at it under a microscope, and provide a positive identity to your culprit. But most remedies can cover ich and velvet.
I would also suggest that in the mean time, you have a "sick-bay" tank for those obviously infected. You should never really treat your whole tank with medicine. It is always better to remove the livestock to another tank, treat them there, and do a water change on your main tank. If it is ich, they need to attach to a host within 48 hours after hatching, or they will die. This could take a week or two for an infested tank to clear. continue to do water changes every few days on your main tank while the livestock is in another. Once your fish have recovered completly, and your tank has had at minimum, a week to cycle, you may slowly re-introduce your fish back into your main tank. This could very well happen many times to you. In one year, I dealt with it 3 times. I found though, that another major contributor of ich introduction into your tank is wild caught fish. As far as health and hardiness go, captive bred are always a better bet. I hope this was of some help, good luck.
--Emi