If you buy a yeast-based kit (eg hagen Nutrafin) it will come with some kid of diffuser, in Hagen's case a grey plastic ladder - this will be hard to hide in such a small tank.
I'd go for the smallest ceramic or sintered glass diffuser you can find - they're often sold as Rhinox diffusers. You can use this with any of the kits, or with your own creation if you've made a DIY reaction chamber. This will be the easiest option. Hardly seems worth getting an inline diffuser or reactor for such a small tank!
Mark