I'm not sure what works fastest, but if you have corals you probably don't want fast. Lowering phosphate too fast can harm them. I can attest that GFO/Phosguard + water changes do work, but over the course of many months.
As for products that help get rid of algae, they may kill it, but those nutrients are still in your tank, feeding the next outbreak. Suck it out during a water change. For GHA, rubber band a toothbrush to the end of your hose and scrub it off, letting the hose sick it out. Do it at least weekly - frequent water changes will also help pull out phosphates. You won't measure a change because there is a balance between phosphate in the water and phosphate captured in the rock. The rocks will leech more phosphate into the water as you remove it. You're only measuring the water, so you won't see a change with your test kit, but rest assured you're taking phosphate out of the tank. Eventually it will start to go down.
Also, budget for lots of salt.