TBS speaks to this a couple of posts up.I purchased 30lbs TBS live rock plus 20lbs live sand and it is in the new setup for a week. There are A LOT of cirolanid isopods, a lot! And they definitely can reproduce, even with a fallow tank. I captured more than 10 tonight, and there are quite a few more huge ones that I missed, not to mention tiny ones that I didn’t see. It is disappointing that this hitchhiker is not listed on their website, while apparently they are aware of it. Looking at all the posts on how to battle them over a couple months is frustrating. Wish I could get all of them out.
I've seen a couple at the beginning of various tanks, including this latest one, and what they say above has always been true for me... they just age out. I don't do anything to try to remove them. This current tank is now 9 months old and doing great, no pests. I saw a mantis recently but other than some snails I would assume, he's not bothering anybody.