How close to the Frogspawn? They have a wide reach with their sweeper tentacles and can cause recession like that.
As for lowering the phosphates there are multiple methods. Water change is the number one choice to bring it down in a hurry. GFO is the second choice but takes it down slower and with regular maintenance will keep things in check. On one of my tanks I had a very high phos and need to get it down in a hurry and used phos pads in the sump for 40 hours, Aquarium Filtration & Maintenance: Phosphate Filter Pads. These are only a fast stopgap measure and not really a means of long term control, I started GFO after using them to get the phos down. Bioplastics are another means to get phos down also. It's made for nitrate control but does bring down phos too. Once you get the phos down watch what you are feeding the tank and how much. Some foods have a high phos levels.
As for lowering the phosphates there are multiple methods. Water change is the number one choice to bring it down in a hurry. GFO is the second choice but takes it down slower and with regular maintenance will keep things in check. On one of my tanks I had a very high phos and need to get it down in a hurry and used phos pads in the sump for 40 hours, Aquarium Filtration & Maintenance: Phosphate Filter Pads. These are only a fast stopgap measure and not really a means of long term control, I started GFO after using them to get the phos down. Bioplastics are another means to get phos down also. It's made for nitrate control but does bring down phos too. Once you get the phos down watch what you are feeding the tank and how much. Some foods have a high phos levels.