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9supratt4 is giving good advice to research this topic thoroughly.
Some swear by it, and others do not like it.
Personally I am undecided.
Since most of what you will read are the positives, I will throw in one statement just made by Steven Pro on a web chat.
His response to this topic was...
just to clarify, are you saying the leg was as long as your nail, or as wide?
One leg as wide as your nail, would be one monster crab.
I have a AccliMate, which can be used as a guillotine trap. I would use something like this, but there may be options others can suggest that would not cost...
Try this site.
Look under crabs and odds and ends.
http://www.chucksaddiction.com/hitchhikers.html
Also note the size of the sea spider on this site.
http://www.lionfishlair.com/hitchhiker/hitchhiker.shtml
It may help you decide if it looks about the size you saw.
if you put a piece of regular raw shrimp where you think it may be, if it is a crab I would think it would come take a look.
That said...you never know.
The only pic of a sea spider posted was not by the person with the issue, but by someone trying to help.
I would not suggest to start tearing my tank up, or dosing chemical treatments, based on the info provided thus far.