Mollusks keep water in a nitrogen balance.
https://www.whoi.edu/oceanus/feature/can-clams-and-oysters-help-clean-up-waterways/
Guess what eats the nitrogen before the mollusks filter and remove it from the water? Mollusks cannot incorporate nitrates unless it is “fixed” by PLANTS (phytoplankton aka “algae”) first. The algae remove the nitrogen from the water, the mollusks remove the algae. Try again...