Fung Heung-lan pointed at a pond she owns in Tai Sang Wai, Yuen Long, saying intervention is needed to prevent the fish in it from dying.“Look at its water – it has gone yellow,” the 70-year-old explained. “There’s not enough oxygen in the pond.”The veteran fish farmer said good quality fish pond water with sufficient levels of dissolved oxygen should be a light greenish blue, as is the case with a neighbouring pond she also owns.Fish need oxygen to breathe and oxygen-rich water allows…