Definetly would not work. A lionfish should never be kept in a tank that small. Even a dwarf lionfish should be housed in at least a 40g+ aquarium. Lionfish are predators, and like all predators, are messy eaters and create a lot of waste. This makes it very hard to keep, even an aquarium as small as a 40g clean, with stable water conditions. I have had a dwarf lion in my 40g and he was fun to watch hunt through the rockwork so I know why you would like one, but use a 40g+ aquarium and let the dwarf lion be one of 2-3 fish. Then you have the problem of finding 1 or 2 other fish for variety that don't get very large. I kept 2 domino damsel fish with mine and he never ate one, but if he did it was no loss to me as I catch them for free.
I think a 12 gallon would be fine for a mantis. When you keep them in a larger aquarium you see them less. I have 2 mantis in my 40g and hardly ever seen them. The ciliata in my 20g is a different story. I see him out hunting all the time. I set-up that tank with only 3 rocks, nice size, but not touching so he couldn't skirt around unseen all the time. I also keep 2-4 domino damsels in the 20g to liven up the tank and for my ciliata to spear. To keep the dominos alive I feed regularly with live brine shrimp to both of my tanks. Both the philippine mantis, a smasher, and the ciliata mantis, a spearer, love eating the live brine shrimp.
I hope this very long post helps a little and that you find a nice mantis to house in your 12g eclipse.
