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Recently our yellow tang had an injury on his snout, near his mouth. We thought either he bumped it or there was something foreign in the tank, that hitch-hiked in with the live rock and it attacked the tang. Tang has healed now, so that seems ok, but last night we found a nasty looking black creature in the tank. It looks a combination of a centipede (spelling??), cockroach & maybe prawn. We do not want this creature in the tank, as we think it responsible for the injury to the tang. It crawled inside a hole in the live rock & we can't reach it in there. We removed the piece of live rock & it is now in a bucket of SW. We are thinking of leaving the rock out of water for an extended period of time until this creature leaves the rock or until it can no longer possibly be alive. Will this also kill the live rock itself? Would there be any problem in returning the dead live rock to the tank, after we are sure the creature is no longer around.

If not, are we better off disposing of this piece of live rock & buying a new piece. The tank is cycled & a flame angel was last week. Is it ok to add new live rock, when we have just added a new fish? Will that be too much for the good bacteria to handle, so close together?

Any advice is appreciated. Thanks in advance.

Cheers
TW

Well there are some rather weird looking centipede creatures(bristle worms) that are quite beneficial to your tank and harmless to your fish. I suggest at least attempting to ID anything living in your tank before declaring war on it.

Eventually yes it will die in the bucket but depending what it is and how hardy it is it could take months. Try getting a large syringe(20cc) and suck up a bunch of water then squirt it in the hole to wash the critter out. If you use RO or distilled it may just kill it in the hole otherwise you can capture it when it falls in the bucket and return the rock to the tank. If it dies in the rock you can use the rock again but would be best to wait for it to cycle again. Drop a small powerhead/water pump in the bucket and wait till there's no ammonia or nitrites coming from the rock. It is then safe to put back in the tank. May take awhile though. Other creatures on the rock may die while the rock is cycling again which will delay the cycle and how quickly you can use the rock.

If it looked anything like this then it is probably a mantis shrimp that you describe. Don't get your fingers too close, and it's a good thing you got it out cause they can be nasty little buggers. They will hunt and kill all crustations and possibly nab unwary fish as well.

They will come out to feed, so if you throw in a hermit as bait, wait till the mantis comes out and quickly remove the live rock all should be good, except for the hermit. I've met some stubborn stomatapods that will not come out in freshwater or no water. They would sometimes rather die, which is odd logic. If one of those died in your rock It would be awhile before it cured again.

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Critical Fertilator: The Micromanager of Macronutrients

Agreed, it sounds like a mantis of some sort. You won't want one in your tank unless you've prepared for it, so this mantis needs to go. If you can get it out alive (I've heard that squirting soda water in its hole will work), you might find a local shop that wants it. There are plenty of people who like mantis shrimp, and would be happy to have yours.

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And when he gets to Heaven, to Saint Peter he will tell: "One more Marine reporting, Sir! I've served my time in Hell."

Yes it is a mantis shrimp. Thanks for all your advice.

Cheers
TW