I also doubt that it is a mantis. I have a mantis, he never bothers my free swimming fish. Clowns, Yellow Tang, Hippo, Damsel, Pseudochromis have all been fine for up to 5 years.
Now, rock dwelling fishes like firefish, gobies etc (and shrimp) come up MIA at times.
Good luck on solving the tank issue. I think that you are on the right track letting the tank go fallow for a while.