I can foretell the answer, but I just have to ask.
Take a dragon with the feat Ability Focus applied to its breath weapon.
Now, there are many dragon special attacks whose descriptive text says:
"The dragon can use its breath to..." and "The DC is the same as the dragon's breath weapon" (examples are the Black Dragon's Acid Pool or the Blue Dragon's Storm Breath).
So, must the +2 from Ability Focus be added to these breath-derived attacks or would they need their own Ability Focus applied?
RAW, the Ability Focus would apply to those additional powers as well, since they specifically state they use the same DC as the dragon's breath weapon, and the Ability Focus feat also applies to the save DC of the dragon's breath weapon.
While that's true, it's also true that Tail Sweep's text too says "DC equal to that of the dragon's breath weapon", with Tail Sweep not being a breath-related effect.
Plus, maybe any single special attack should be considered an "entity" of its own, thus requiring its own dedicated Ability Focus, even if it is "derived" from another. Otherwise a, Blue Dragon (random dragon for example purpose) *could perhaps* have the feat applied to its breath, giving the +2 DC to Storm Breath too, and take a second Ability Focus for Storm Breath, adding another +2, for a total +4 (the bonus should not stack, normally, because it must be applied to a new ability every time the feat is picked, but technically in such situation it'll have been picked for two different abilities, the second of which augments the already-augmented DC of the first one).
Well if it is directly related to it's breath attack it is fine, I would not use the ability focus for the tail attack.
The more I think about it... the more I think that Ability Focus (breath weapon) should ONLY apply to the breath weapon. You want to have that +2 on Storm Breath too? Take Ability Focus a second time.Does this mean that there cannot be an "Ability Focus (Storm Breath/other breath-related ability)"? Or can it be taken and stack (or not) with Ability Focus (Breath Weapon)?
Ability focus applies to one ability. In the case of a dragon, his breath weapon is that ability. As many dragons get older, they can use their breath weapon to cause additional effects—this is still his breath weapon doing the work, though, so his ability focus would count.
From this thread.