GH/KH = soft?
Dosing Tropica Master Grow?
What else are you dosing?
Information might help to help you find a solution. Here is my own account:
I had a thread algae problem emerge every time I dosed TMG. The water I have is soft to the extent there is very little readable GH/KH. I eliminated the thread algae by dosing H2O2 (hydrogen peroxide) but it returned every time I dosed TMG. This was an irritating problem in that TMG was giving good quality leaf growth and I wanted to continue using it.
I was also dosing KNO3 and PO4 (via Fleet solution).
For various reasons, I started dosing an amount of K2SO4 (potassium sulphate) equal to the KNO3 I was adding. Possibly, over months of dosing only KNO3 and PO4 a K shortage had come about - although I did not notice any particular slowing of plant growth. Anyway, the result of dosing K2SO4 was the demise of thread algae within days (no need to use H2O2 either). Plants pearled vigorously almost immediately I added K2SO4. The thread algae has not returned and I am using TMG regularly now.
Possibly, my solution has some bearing on your problem. Possibly not.