Well, phosphates fuel algae, which means something is causing the algae, I think it would bbe overfeeding, or the left over food is not being consumed, there are a few options you can try. lower the amount of food entering the tank, this will stop the uneaten food causing algal blooms, clean up crew is another way, i had recent problems on my sand bed aswell, and quickly fixed it with a sand sifter goby, they are good because they clean you and keep the top layer well oxygenated, also and uneaten food is discovered by them and they will eat, also many have good character and are fun to watch, also other options are snails, hermits, starfish etc.