You did not say how big your tank is & what kind of filtration you are running.
I battled that stuff for months & realized I was way underpowered in filtration. Running twin canister filters + carbon to knock down built up DOC's helped. I have only a 10 gallon. Also accidently treating the tank with the wrong meds, in this case flagil for a parasitic problem that I may never had. At the same time I decided to do 1 day semi black out, which I did.
With each step I watched the BGA slowly fade away. I had a really viral stain that was very resistant to anything. But the added flow & filtration & maybe the drug combination helped a lot. I also changed my lighting to 6500k which BGA does not like. It has a very narrow window in the spectrum that it cannot thrive in & 6500k is it.
I would have to say that white sand is the worst choice for substrates. BGA loves that stuff because it creates an anaerobic compacted substrate that low in O2, it thrives in that stuff. Go to u-tube & watch all the poor soles trying to get it out of their tanks & they all have that stuff in their tanks.
I know because I had some in mine & BGA thrived where ever I had it in. So I removed most of which only made about 5% of my substrate the rest is Flourite.
Filtration & good flow will help beat it down. But do not stop fertilizing your plants & do not raise your lights. BGA will only find it even easier to grow. You have to provide good plant conditions as well to beat BGA. Keep NO3 above 10-15ppm, at least. From my experience Fe should be kept a little low during this period , but do not limit nutrients over all.
Not to beat it to death, but I run a Fluval 106 & Zoo med 501, but it would not be overkill to throw on a small Ehiem 2215 or 17 during this period & you might even want to leave them running for the life of the tank.
On a closing note: The Black out alone will not always work & stress your fish out & your plants. The medication alone is not reliable. Also if you do not fix all problems mentioned. Lighting, filtration, low NO3, proper flow, substrate vacuuming & proper nutrient dosing leaning towards EI dosing , then it will return. Hope this helps.