tadpoles have been known to have a large population in small areas. the more room the better that is rule of thumb.. if they are a small species that you expect 100 in a 15 gal tank may be alright IF you can properly maintain cleanliness. a 20 gal tank may be more optimal if you can afford it they are very cheap. they need comfort. (plants for hiding places.) If i were you i would transport any unwanted tadpoles to a nearby creek, river, or pond. or you can keep them until they morph and release them in the environment near where you found them. many may die of natural causes big reason why frogs lay so many eggs. My good friend CJ on this site raises hundreds of tadpoles a month he can give you the best info on your case. Much better info than i can.