I look at it as the algae locks up the nutrients as opposed to "using them up" in any case the nutrients arent available to be used hydroponically without conversion. But that is why using pond water on your soil garden or lawn can be so productive because the algae can get broken back down into nutrients in the soil. And it's effectively removing nutrients from your pond (if that is your goal)

if i was going to try something like this, i might base the design of of mark mcmurtry IAVS system which uses sand based grow beds. The sand can filter particulates (algae and solids) from the water and get broken down then provide some nutrients. Not really the designed application but i bet it would work.