I got rid of all of my hydra in my shrimp tank by lowering the water level for a week. It increased the flow and while my shrimp didn't really enjoy it, I have not seen another hydra since and it's been about 4-5 months. I think extra flow really helps. I had tried manual removal, feeding less, and lights out before but they always came back or never completely left. The flow thing seemed to have done the trick though.