Becuase this form of speciation consists of physical separation on a very small level, it is also called micro-allopatric speciation. As was the case with allopatric speciation, once the two populations are separate, their gene pools are free to change independently of each other. Genetic drift is a random process that can result in the relative levels of alleles going up or down, and has a much larger affect in small populations. I think that this could be true under certain circumstances. In particular, I think that you could have a sympatric speciation if a particular behavior resulted from the expression of a neutral allele, and if genetic drift was on your side.
Lets say the size of the fish determines where they swim and what they eat.
Big fish eat large bugs on the surface, medium sized fish eat things swimming in the water, and the smallest fish eat tiny creatures that live in the mud at the bottom of the lake. Accept that there is plenty of food to go around, and that size doesn't matter as far as reproducing goes. Now, suppose we have a drought, most of our lake dries up, and two groups of fish survive in two isolated pools. The survivors in one pool are all small, and the survivors in another pool are all large. Eventually, these two populations could diversify to the point where inter- breeding is impossible, but the initial barrier to reproduction would have been due to the effect of dramatic genetic drift on a neutral allele.