i'll bet they are fish fry. if you have live bearers (like platties, mollies) then they give birth to live babies. they retain the fertilized eggs in their bodies and then when they babies are ready to come out, they come out of your female alive. another possibility is that your fish had laid eggs, you never noticed and then the fry "hatched". but that wouldn't really be plausible, well unless you don't vacuum your gravel every two weeks. you should be doing that twice a month, but if you have only two tropical fish, then yeah, i suppose if you vacuumed once a month i would understand. specify what type of fish you have, so then we will be able to know if you have live bearers or not.
