I want to do something with this background and the sea nettle + fishes but I think that the clothes of my HA seems to look odd :/

Can someone suggest something for the clothes?

I actually really like your HA. I would suggest changing the dark blue tank and perhaps changing the hair but other than that I like it

The blue and the orange are very bright and look muddled when put on top of one another in your outfit. You need a color in between to help them blend. Try a black base for the clothing with pieces of orange and blue, that might work