Fish

I am a fish.
I may call myself the one without past.
My memory will only last for seven days. After seven days, my memory will start again, and I won't remember anything before.
This is my seventh day. I won't remember anything tomorrow.
Someone told me that I would be happy that I won't remember sad things because of my short memory.
But actually I felt lonely. All of my friends won't remember each other. I don't have friends or family.
I do everything alone.
I am going to bed now...
It's a new day now.
Who am I?