Question

water is a compound so when fishes take in water and separate oxygen from the water so how can they release carbon dioxide, when oxygen is separated from the water  the left molecule is hydrogen.then it should release hydrogen. ​

Answer 1

Aquatic organisms do not use oxygen that is present in the molecular composition of water I. e. H2O. They use free oxygen that is dissolved in the water. The oxygen from is atmosphere is dissolved in the water which is then used up by aquatic animals. The oxygen is absorbed via organs like gills which are in constant contact with water. Gills also have higher surface area that favors diffusion of oxygen.

so the metabolism is usual as in other animals, carbon dioxide is released as a result of respiration (not hydrogen) .This carbon dioxide released is then dissolved in the water and later emitted out into the atmospheric air.