Ocean mobs won't swim

I was building an aquarium at my zoo when all of the peaceful ocean mobs such as fish and dolphins were sinking to the bottom

 

What I expected to happen

All the ocean mobs would swim everywhere

 

What actually happened

They all sink to the bottom

 

Steps

1. Create a super flat world that in creative
2. Go to bedrock level the build up 7 blocks
3. Spawn in any ocean mob like a squid or turtle