If you do put a heater in your tank get one that's not made of glass (same goes for the thermometer). They like to tap on things and could possibly break a glass heater/thermometer.
Happy mantis keeping! They're oddly endearing little creatures aren't they?
I'm setting up a 2.5 gallon reef in a couple of months. I'm planning on buying an AC20, would that provide enough flow or should I pick up a power head too?
Cool tank! I was going to do the same thing with my 2 gallon eclipse, but then my betta got kicked out of his old home so I gave the tank to him.
That's really sweet that you bought a clown with a lucky spine.