Phosphate sponges are ok, but you have to kill out the source of the phosphate. Make sure that you use only RO/DI water. Always! If there is any other kind of water in the tank now, I'd do a water change with Ro/DI water. Also, how much are you feeding and what are you feeding. Overfeeding will put too much phosphate in the tank. I feed my fish ever third day and they are fine. Also be careful with your coral food feedings. This is more of a nitrate problem though. Problem algae like green hair can only survive and grow if they have the nutrients to do so. Your algae is getting it from somewhere. By the way. Snails that are a couple of years old in a tank? Cool. They probably died of age. If other tank parameters are ok (ph, salinity, and calcium for snail's shell growth) they probably were old. For hair algae. trochus snails in my opinion are top dog. Pich up a few more of them. Also, diversify your snails. Get a few of each species because they will inhabit different niches etc. What I mean is that big trochus snails will not be able to go in smaller places that little snails can get to and be effective. One more thing, a blue linkia starfish will destroy a hair algae situation quickly.
