No ich treatment can be successful with just a single dose. I definitely would not buy a product that claims it can because the manufacturer of such a course of treatment doesn't even know the basics about ich itself.
For the absolute best article on ich on the Internet, read
<a href="http
/www.skepticalaquarist.com/docs/health/ich.shtml" target="_blank">http/www.skepticalaquarist.com/docs/health/ich.shtml</a>
In particular, pay attention to the segment on ich's lifecycle. Even more specifically, the section where it talks about how ich is only vulnerable to medication during one of the phases of its life cycle. A one-off treatment will only kill those ich that are in the vulnerable phase. If you only treat once, the ich that was not in the vulnerable phase will eventually grow up and attack the fish again, so using a one-off ich treatment pretty much guarantees your tank will have ich again in the future. (if you are a conspiracy theorist, you may belive the manufacturers actually do know this and it forces you to come back and buy more medication time and time again.)
You have to medicate for ich every day until 100% of all signs of ich are gone. No more white spots, no more flashing, none whatsoever. Then, you have to medicate at least 4 more days (at tropical temperatures). You have to do at least four more days to make sure you get rid of 100% of the ich population -- in case a few survived the first round of medication and your fish just aren't showing symptoms anymore. If the fish don't look stressed out or weakened from the course of medications, I'd really recommend you medicate for 8 or even 12 days after all signs are gone. You really, really want to ensure 100% kill of ich. Because, even one organism left alive can grow back into a large population and then in 3 months you'll be back on the forum complaining about the dormant ich in your tank. It wasn't dormant, it was you not killing even single last one. All of this is covered in that link above, too.
There is one medication that may be good in a one and done treatment, and that is a copper-based medication that stays active in the water until you take it out. The trouble is that the copper based medication kills a lot of things, snails, and even fish. Some species of fish are very sensitive to copper. Also, you have to remember to replenish the copper every time you do a water change. I also don't know if copper is really all that effective against ich.
The formalin-malachite green mixture is very effective against ich, however. But, it has to be re-dosed every day. It loses its potency pretty quickly.
So, just to recap, re-dose the ich medicine every day until symptoms disappear, then continue to re-dose every day for at least 4 more days, preferably 8 or 12 if the fish seem like they can take it. You have to kill every ich in the tank to ensure it never comes back, and ich is only vulnerable for a few hours every 3 to 4 days. So, there always has to be fresh and effective medicine in the tank so that when the ich change into the stage in it's cycle where it is vulnerable, it dies.
