I'd be more worried about the mantis biting back after getting stung. A galss aquarium would be ok, they never broke any of mine. Most anemones get too large for a 20g tank. Only ones I would consider are condy anemones. But if you get those, you can't keep corals. The reason being is cony anemones move around. A LOT. Put them one place, and an hour later is on the other side of the tank. It could kill or stress any coral it touches. But if I were you, don't keep corals. Especially with a mantis shrimp. Most corals at the very least are 15$ and adove, and that's only for a 1 inched mushroom. You'd be waisting money keeping hard corals as well, they need STRONG light and lots of current, others like polyps and mushrooms wouldn't do well with a mantis.