Mermaid Melody Pichi Pichi Pitch, Princess Tutu Recommendations
1: They both have the tranforming girl(s)
2: There is romance
3: In Mermaid Melody, If a mermaide tells a human that they are a mermaid, they turn into bubbles. In Princess Tutu, If Ahiru tells Mytho that she likes him, she will turn into light.
4: The art was similar
They are both Magic-Girl animes.