A little lesson in physics,
Its all about the suction, suction, suction not the flow! Technically there is no such things as suction just a greater pressure on one side than the other. High pressure areas want to move to low pressure areas. The problem is that in a pipe you only have one intake and a constant pressure less than the ambient. If that one intake is covered the pressure inside the pipe will be much less because of the pump. This causes whatever it is stopping the flow to stick. So the solution is to have multiple inlets of water. That way it always has somewhere else to take the flow from so the pressure the pump is exerting (which is constant) is essentially divided by the holes. That way it is the same amount of flow but much much less pressure. You will not have the problem of a fish getting stuck if the pressure(force) is not great enough to keep it there.
That is why I proposed the extended pipe with hundreds of inlet holes that way the pressure is very minimal but the flow (gph) would still be the same so you would never have the problem of a fish getting stuck. Not only that but the holes keep larger living things out of the filtration.
Hope this helps