Indeed. I know you say you don't want a script so I'll keep this under 10,000 words but too many people are going this type of route.
Use a chemical to kill the problem. Don't know what caused and guess what 'Hello I'm back'.
This is true of people using excel to kill algae and also things like this.
Sort out the disease not the symptom of the disease. If don't sort out the disease then the symptom will come back.
I would do as suggested above. Clear out most of it, do a full 5 day blackout with absolutely no feeding or looking. Wrap it up in thick bin bags, several layers if necessary and tape it all up. That way you will not be able to take a sneaky peak.
I would bet that your new filter
may sort the probme out althoughyou shouldn't need the UV. I used to have one and yes it kept the water clear but guess what. I sold it and I still have clear water. Namely the disease isn't there so I have no problem.
There are disputes over UVs being a problem with feritlisers namely that they weaken chelators in the traces. No idea how factual these are as I ditched the UV as I saw no use for it.
If the blackout and filter doesn't sort it out then you need to look at ferts namely nitrates being too low.....yes I did say too low
Sunlight can cause it to start too. Some people put black insulation tape along the visible sides of their tank at substrate level as you can often see some BGA there. I can see some in mine. but then I have seen it for over 3 years without tampering with the substrate and without it spreading so I don't worry. If it does spread for any reason you can bet your aunt I won't be buying any treatments for it
Told you I'd keep it short. Thats really short for me
AC