Melev did a great video on YouTube about removing GHA:
https://youtu.be/xilqKzoOgBE
I also fought this battle, while I agree with the others here in terms of removal, you also need to tackle the root cause of it which is usually an excess of nutrients.
A reduction in nutrients has visible effects on GHA. Within hours of hitting trace numbers of phosphates it looses its colour and also looses its grip on your rockwork... It also makes removal so much easier.
Depending on how much rockwork you have, removing it entirely by hand can be near impossible as healthy GHA grips the rocks so well. By tearing the majority of it away, it's rooting system is still usually present meaning that if phosphates are readily available it will simply grow back. I've been here and have felt the frustration of it.
You have a few choices...
- As others have stated, buy a chemical that targets the algae directly.
- Work with GFO and good husbandry to reduce your nutrient levels gradually
- Use other kinds of chemicals that bind to phosphates, similar to what Melev does in the linked video.