FOS is fructo-oligosacharrides, a prebiotic that has been shown to increase akkermansia counts in human studies. It also seems to feed quite a few other species, so I'm not 100% confident it will work. Here's the Amazon link to what I'm using.

I plan on sending in another sample in 3-6 months once I've had a chance to feed them and let them grow, so we'll see! I've had good success with prebiotic supplementation in the past.