I would strongly suggest finding out which "antimicrobials" are found in the different herbs before trying them since some of them use saponins and glycosides as thier antimicrobial molecules. Both of these are likely to make the tadpoles fairly unhappy (and some saponins are known for thier toxicity for fish....)I stumbled on this article and that it was extremely interesting since I myself drink this tea on almost a daily basis.
It makes sense but then the question I would pose is why can't other antimicrobial teas be used. Like mint or Yarrow for instance they both have antimicrobial properties so would those work as well?
Great idea though to help keep the germs away from our little babies