how did fish evolve to breath on land?

There is a structure found in some fish called a gas bladder. It is an outpouching of the digestive tract which is used by some fish to maintain neutral buoyancy. The lungs of tetrapods (amphibians, reptiles, mammals and birds) derived from a comparable embryonic outpouching (homologous structure). In the lungfish, which can breathe air, the same structure is a primitive lung.

Some fish have gas bladders that help them with bouyancy. Some fish have vascularized gas bladders. There's not much difference between a vascularized gas bladder and a lung: and some fish have lungs: lungfish. Some fish have to come to the surface to gulp in air every 10 or 15 minutes: Piraracu (Arapaima gigas). Some fish even spend lots of time on land: mudskipper and mangrove rivulus (up to 66 days).

I was once a fish! just thought i'd mention it. I think maybe i procreated with a lizard!