I think you could do that assuming you can control the flow rate and amount. It would be a good way to flush the small pond of excess ammonia and nutrients. You would need to be careful about using it during the spawn so as not to muddy up the water or silt in the beds. If you do then be sure to siphon in water from the top foot or so of the big pond (high in O2 and plankton) and not pond bottom water. How will you get rid of the excess water in the small pond? If you can take out bottom water from it.