Welcome to AA, Damien!
She seems to be quite distressed and was constantly trying to straighten herself up, but to no avail. It has now come to stage where she is worn out and just floats belly up in the corner or trapped between a rock. All the other fish in the tank are fine and still very healthy.
I've searched for other posts about this, but just wanted some more advice. Do you think its the swim bladder and do you think i should try peas??
My gut feeling is there is a problem with the swimbladder. She is also an elderly fish I am sorry to say. Just as with people various systems start to break down as fish age. Take comfort in knowing you gave this platty great care during her life and keep her as comfortable as you can.
I highly recommend peas once a week for any fish that will eat them. When it's pea night at my house, that's all the fish get for the night. My community tank, Mbuna cichlids, and Mollies love them. I use frozen peas, microwave them, peel them and serve!
Since you are new to the board, I will also ask the standard Qs~
What size tank?
What/how many fish are in there?
Water parameters (ammonia, nitrite, nitrate, temp, pH)?