just to clarify and not confuse anyone, clownfish do not "change sexes at any given time" (which to me implies they can switch whenever they want to)
All clownfish start out as unmature juvi males.
In time, one clownfish will out compete the other(s) for food, grow larger and dominate its territory...that clownfish will then turn female.
Once a clownfish turns female, she can NEVER turn back to male. This is a one time sex change.
On the other hand, any male (either juvi or submissive mate) can turn female once the dominate female clownfish either is removed or dies.
For this reason, it is always recommended to introduce a small clownfish as to better insure it is male.