Dolphins not going up for air

We recently built a survival aquarium, and all of the 10 dolphins we put into a huge tank, with all water source blocks and an open roof top for air, committed suicide and didn't swim up to the top to get air. All dead after 10 minutes and it took forever to get them from the ocean. To recreate, make a large tank w all water source blocks, and put dolphins into it. Our dolphins never went up for air and died