I 4got the movie title, a mermaid that can tranform i2 a girl when not in water.

2 friends found a girl in a cabinet. naked. which was a mermaid who transformed into a human because there was no water. and that mermaid had colorful hair, which, those 2 friends became her bff. then the 2 friends cutted the mermaids hair to look like thiers too. and the mermaid gave her bracelet to the old man, then she swam on a water tank. I only know this story..

Could it be "Aquamarine?"

http://en.wikipedia.org/wiki/Aquamarine_%28film%29