I have a three gallon mantis reef at the office and it has been remarkably stable - the water parameters have always been excellent whenever tested, more stable than my larger reef. Just be sure to keep up with top offs. I have lots of live rock, corals, good water movement, and my mantis is the only inhabitant. The corals are even growing. This tank's been set up for nearly two years. My only regret: putting mushrooms in a tank this small - they're about to take over the world and you can't get rid of them!!