garlic can help with the prevention of parasites
the garlic istelf, if not removed could be a minor problem, since it is organic, it may rot, whihc could contibute to a ammonia or nitrate problem(temporarily, and probably minor, considering the size of the piece)
this is one very good reason to use kyolic extract, it is more concentrated, nad odor free, and has no whole parts whihc may get lost in the tank, besides, it will last until used and is relatively cheap, considering the fact that you only use a couple of drops at a time
HTH and good luck