Snails are great for fry tanks - they clean up all the little bits of leftover food, don't create a huge bioload (but enough to keep the filter bacteria happy) and are completely and utterly harmless to fry. Best of all, you can often get them for free or very inexpensively from your LFS - many plant tanks are teeming with ramshorns and they'll happily give you a few if you ask. If in the future you shut this tank down, snails can easily be transferred to another tank, where they'll eat algae and possibly provide a meal for other fish (if you have loaches).