I had a mature tank that went cloudy like that. I would do daily 20% changes and then the very next day it was back, sometimes worse than before. Plant growth ceased, some melted and even some fish died.
It started because I cleaned to much of the filter media at once, had really high light with not enough/consistent CO2 and kept 'adding chemicals'.
I was trying to 'fight' the water instead of working with it.
In the end I was able to stabilize the CO2 levels, rearranged the filter media, re-stocked with plants and let it all settle for a few days.
I stopped adding chemicals to try and counteract changes in water chemistry and just let the tank sort itself out.
Purigen is also fantastic at clearing water.
Hope something in there is of some help.