I’m currently battling GHA myself and I’m not sure of the cause. I did have a cyano outbreak last month and treated with chemiclean. It eradicated the cyano however that’s when the GHA took off. The tank is almost 2 years old and I have always had zero to little PO4 and NO3 readings. I did a deep cleaning of my sump 2 weeks ago and vacuumed a ton of detritus. I added turbo snails, 2 pincushion urchins, 10 hermit crabs and 3 emerald crabs. I baste the rocks every few days and manually pull what I can. My corals are looking great but I’m afraid the algae is going to suffocate them as it’s starting to grow on and around them.