under gravel would be the way to go, the only 'ugly bit' about it is that theres a clear tube that sticks out of the gravel. you might be able to get away with trimming a square one if you cant find a round one. or you could use something like this:
http://www.trademe.co.nz/Home-living/Pe ... 201502.htm
not too expensive, easy to clean.
can yopu take a photo of the bowl? seems big from your description