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I've heard of people keeping crabs in the sump, but it might be hard to find them and fish them out. Also, if you use fiddlers, they are brackish water inverts and also need some time out of the water.
I've found them very easy to keep in a big wide and low fishbowl with some rocks and shells forming a little island and half distilled water, half saltwater (inch or two). You have to change the water every couple of days, but just takes a couple of minutes.
another reason for having them in a seperate tank is that they are adding to the tank's bioload... EG they are producing a lot of waste and adding that to the tank and putting more stress on the filters and skimmers... and belive me... octo live food, like crabs are messy!