To recreate:

1 - Go to an ocean with a water bucket, and pick up lots of fish. (fill your inventory with fish buckets)

2 - Release these fish into your own water source 

3 - Then go back to the ocean, and you will notice all the fish are gone.

My advice for anyone now, don't make an aquarium yet until this bug is fixed as the only way you will ever get any fish back is if you kill all entities using /kill @e which will destroy any item frames, minecarts, passive farm animals and any other entities in your world, which is a pain to get back again (I had to do it).