What is needed is not a gravel separator. When cleaning an R&G (rock and gravel) pond the aim is to remove as much mulm, sediment and debris WItHOUT removing the gravel. This is where arises the problem....removing this predominantly organic material but leaving the gravel in place. This can easily be accomplished with the pond drained but is close to impossible while the pond is full. Any vac with power enough to lift sediment laden water will also lift gravel.
"Everybody is ignorant, only on different subjects." - Will Rogers