A few questions that might help us understand what is going on.
Is there wood in the tank ? What is your normal water change routine?
What is “perfect water” and “everything was normal “ . Ppm
would help me.
You write “all food is always organic “Does that mean you don’t feed algae pellets at night. With a few blood worms from time to time.
Sorry I have not seen organic fish food in Australia.
How big is she? At 12 months they can be 4 inches or more. Is she wide at the shoulders . A photo might help.
I come from the school of thought that 50% is a maximum water change. Followed by another a few hours later in an emergency.
Without wanting to sound rude why would you do a 75% water change if you change water regularly?