If algal blooms are the problem perhaps sinking some night lights down to exercise PAT – Passive Aeration Technology where the heated water rises and creates currents while the lights stop the algae consuming the oxygen…
Ever felt feint in a shrimp RAS facility?
Closer to ground higher the concentration.
Of course even Papillon would cover his mouth at night to raise his carbon dioxide levels and help knock himself out….but would wake up hung over.
BBC News – Four Indian men drown in Italian farm manure tank near Pavia