I have a large male OS (peacock) and i keep him in a 20 gallon tank I built myself with acrylic. I have a little HOB type filter w/ some macro algae inside, carbon, and a powerhead. To be honest with you it is the most challanging tank to keep in check. Mantis eats alot and therefore wastes alot, he is constantly killing snails and crabs (good for him, bad for water paramaters) I can keep it in check with water changes, but I have had hair algae and cyano problems in the past. If I had an extra skimmer I'd run it... I am still tossing around a DIY skimmer idea for that tank as well.