How do you get the koi fish and they mystery fossil fish?

You can get the koi fish by merging the water droplets that come out of the Arabian Garden. Eventually these merge to create a fountain, and from there you can merge until you get to the koi fish.

The mysterious fossil fish can be found by first finding mysterious letters from the Bucket. These merge to create the Map and the map merges to create the Island. The island will then produce the mysterious fossil.