isopods!! need help

second time i've found one in my tank in the last two months, they are pretty drcent sized, clownfish was losing its color so i got it in the net and plucked off the potato looking bug right away, i only find them swimmimg in my tank at night on rare occasions. how can i get rid of them without damaging mushrooms all ofthe stuff natural growing on my rock?? open to all suggestions, thanks!

 

http://s1155.photobucket.com/albums/p555/rus10away/?action=view&current=74f66c41.jpg#!oZZ7QQcurrentZZhttp%3A%2F%2Fs1155.photobucket.com%2Falbums%2Fp555%2Frus10away%2F%3Faction%3Dview%26current%3D74f66c41.jpg
you can see the bruise behind its iowa where the isopod was

 

Here you go folks, finally caught one of the evasive little bastards, well I probably shouldn't say little, this thing was HUGEE! He had the nerve to attack in daylight, which is very strange because I have never seen one out during the day.. and it kind of worries me because it might mean they are getting even more desperate to feed. I took me forever to catch the fish so I had to put all of my live rock and polyps in a corner in order to net Humphrey (my clown, yes I named him). To my horror, after taking the fish out of the net I didn't even have to pry the isopod off of his fin, the thing jumped off and started crawling up my arm! This one is easily 3 times bigger than any other one I've seen sucking the blood out of my fish. I might keep him a while to see how fast they multiply and what else they might eat. But for now, I am certain there must be more in my sand and live rock and am still as helpless as when I first saw one a few months ago.
Here are some pictures of it, not something you would want swimming around your tank to say the least.
http://i1155.photobucket.com/albums/p555/rus10away/419e1e10.jpg
http://i1155.photobucket.com/albums/p555/rus10away/ce303bd5.jpg
http://i1155.photobucket.com/albums/p555/rus10away/30542f00.jpg
http://i1155.photobucket.com/albums/p555/rus10away/d1e568e2.jpg