To me, the answer is.....it depends ;-) Some people I know on here keep a pair of Ocellaris Clownfis in a 10 gallon tank and they seem to be as happy as can be. Other sites state a minimum tank size for a single Ocellaris is 30 gallons. It all depends on how much open space they have, how much bio load your system can handle, the size of the fish, the physical size of the tank, etc. It all depends ;-) Some fish want more vertical space, others more horizontal. I kept a blue tang in a 40 gallon long display tank once when it was about a half inch long (tiny sized). It had plenty of wide open space and length for its size, but they can grow up to a foot long so long term, it would have needed a lot longer tank. Once it out grew the 40, I traded it to someone with a much larger tank. So, short answer...it all depends ;-) As with all my posts....just my 2 cents