Haha, good question. They aren't recomended for tanks under a 10 (at least on LiveAquaria but I get the feeling thats the smallest they recomend either way even if its ok in smaller)... but I would be willing to to put one in it, as they stay small, and as gobies they aren't hugely active fish so less space shouldnt be a problem. So by in theory I mean it shouldn't be a problem, but there is always the possibility that there will be a problem (like there is always a possibility with any fish in any system).