Well, you can see that they do have tails in the photo and the reason I mention it is that you wouldn't be the first to mistake them for larvae. Almost every one I know has done that, including me. Your description sounds like them. So I suggested that you catch one and take a closer look at it. They are really cool creatures in their own right and well worth the time spent watching them and are well worth your time to catch them and raise them in another tank.
Fish larvae do not persist in most tanks, especially not tanks with any kind of filtration system. Right after birth, they are designed to float in the plankton for days and weeks at a time. They don't hide in the rockwork during the day. They are _very_ poor swimmers and can only swim in very short bursts. Fish larvae do not swim like fish. And everything, including their parents, eats them.
So, what you are seeing is probably not larvae.