I have one currently in a 40 and have been trying to decide if I should upgrade this summer. He's 11" now and does swim around, but I would call him an active fish. I currently have 2 HOB Marineland penguin 200 's. Also, the last few times I have checked the water, everything looked pretty good (being no traceable amount of nitrite or nitrate and app. .05 ppm ammonia)