Suppose these two creatures in the dream represent two abstract things, two choices, or two ideas. It seems that you are accepting two conflicting ideas at the same time, and it is difficult to merge and conflict in your mind. Maybe you can't have it both ways. Do you choose turtles instead of fish, or do you choose turtles instead of fish? It depends on your personal preference. My personal opinion is that fish's teeth are too sharp to tolerate other differences.
Finally, suppose that two kinds of creatures represent a part of your heart, maybe you need to have a look inside, how to make the fish less unsafe, maybe you can feel at ease without showing your teeth? Or learn how to live in harmony with such fish. Does it need comfort or does it want to take beneficial nutrition? How to satisfy the inner fish and make it happier? How to make the tortoise stretch his head with peace of mind, do things without compromise, and take good care of himself and others?
Some humble opinions, I hope to help you.