Wow I'm impressed, I tried to research this for you and got nowhere near an ID. It certainly looks similar to me too based on appearance alone. Have you seen this article on that genus?
http://reefkeeping.com/issues/2006-02/bp/index.php
It's definitely possible the one in the clownfish's gill is the same isopod you saw on the tang because they have different stages of development (fingers crossed it is!). I've spent some of this afternoon researching about isopod's and from what I've found, this is a really nasty problem - if there are more in your tank, losing all of your fish and not being able to add more for months is a strong possibility :(
The good news is that there needs to be a male and a female isopod to breed unless it was a gravid female and she has released her babies. But if that was the case you would know by now as you would have seen more than one hanging off the fish at the same time and they would be smaller (according to my reading... lets hope its right!). They can also change sex but I don't know how long that takes to happen and it depends on the species (some species can go from male to female, and other sp. can change from female to male). Night time is the best time to check the tank to see if there are any on your fish, as they can drop off and hide in your rocks or substrate during the day.
Manual removal seems to be the main course of action for isopods.
The best way that I could see to do this is fresh water dipping. Its quick, easy and the least amount of work - also reasonably thorough. You won't be able to do that to your substrate (at least, I wouldn't want to personally), so if any are hiding there thats going to be the hardest area to get them out of. You can fresh water dip live rock, corals and your fish. I've fresh water dipped corals (soft & LPS) and LR plenty of times and its an excellent method of getting critters out of live rock without killing anything, unless you leave it in there for way too long. I have reservations about putting a fish through the stress of being caught and then fresh water dipped unless you know for sure it has parasites or are moving them to QT anyway, but this is also a serious problem so its a hard call to make whether to take preventative action just in case or take the gamble that there are no more isopods in the tank.
If you do decide to pull them out into QT I would definitely take the opportunity to do a freshwater dip using Methylene blue and/or Rescue Remedy (1 drop) to help reduce stress.
If you found any articles helpful while you were researching this, post them in the thread because I can see other members having the same problem sooner or later that will be looking for info.
I've had an isopod bite my leg while night snorkelling and I screamed like a little girl and made my friend take me back to shore because I was too scared to go back on my own... now that I know more about them I'm even more freaked, they're gross! Not one for the rescue tank @firebird ;)
