Sunken side of my gsp

[Pufferkeeperthe 8th]

I have a gsp in a 20 gallon grow out tank with the parameters are at stable, drift wood and dry coral rock for the ph level, the ppm is 1.05 which i am planning to upgrade to marine parameters.
The substrate is a mix of black sand and cocopit or cocofibers. He has guppy tank mates that i have quarantined for a while. I feed him baby shrimps and many different pond snails.

For the most part of my observation is that his behavior is very normal because his colors are vibrant and his swimming behavior is normal, there are no wounds of any kind and he has a white belly. He is eating all the time.
The only thing that is abnormal is the left side of his tail. He has a sunken side of the tail whish every time I feed him that part of the tail is not fatening up.

My best guess is vitamin deficiency but i am really not sure.
If else is missing please let me know and I need suggestions to do to fix the problem.
( I cannot give an image as of now because i am at school)

[Pufferpunk]

There's really no way to tell without a photo.