In rec.aquaria.tech Nikki Casali wrote:
snip
It's a shame there aren't any dimmable CFL ballasts in existence
anywhere in the known universe. Maybe someone can hack one together from
a linear T5 dimmable ballast? I presume there is no electrical
difference between the two?

There are, actually.
Apparently noone makes a "in one" controller chip for 240V, only 110V.
I suppose you could buy dimmable 110V CFLs and use a transformer.