You send them with algae and photosynthetic bacteria, which are also pretty robust. Algae is (are?)a mazing. I would not at all be surprised if you sent tardigrades to enough remote planets, that they would find some local equivalent of algae that is mostly digestable. And photosynthetic bacteria were some of the earliest forms of life on earth, likely surviving adverse conditions (I think it's still an open question whether the earliest organisms with metabolism used geochemical energy or photosynthetic energy)