Fish contains gills which will take the dissolved oxygen present in water.
thats why fish opens its mouth some times when it is in water to is not an amphibian to beath in water and on land because it dont have lungs to breath in.if any living thing want to breath in air it must contain dont have lungs.
A fish can't Breath when taken out of water because of its gill. a fish gill is design to take oxygen in water but not outside the water because of this a fish cant breath out of water.