When my 4.5 yr. old tried this costume on for the first time, she looked at herself in the mirror and said "I look fabulous!". And she did. She even won a prize at the local community center Halloween event. I had my doubts about how well the tulle at the bottom would hold up, but it's still in perfect condition after multiple wearings. It's also surprisingly comfortable for my daughter to move around and play in.
I was on the fence about which size to order as my daughter wears anything between a 4T to a size 5. I ended up choosing the toddler size (2T-4T) instead of the size 4-6 and I'm glad I did. She is 38 lbs. and 42" tall and the costume fit her just right. Unless you have a very tall toddler, those fins are going to be dragging the ground.
My only complaint about this costume is that the green sparkly section of the tail is very thin. If my daughter is wearing printed underwear, I can clearly see the pattern underneath and if there's light behind her, I can see her legs. Our weather is pretty cool in late October so I layered a purple leotard and tights underneath, which did help. But overall we are very happy with the outfit.