Nutrient control is your other option. It takes a long time to get under control, and it won;t totally go away without manual removal of some type. But, a phosphate remover such as phosban or rowaphos, controlled feeding habits and possibly even live rock "cooking" will help out. Also you could consider a small refugium hooked to the back to grow competitive macro algae that will hopefully out compete the Polysiphonia. In the mean time manual removal is your best bet. I would also invest in some Salifert Flatworm exit to help get rid of those flatworms on oyur rock
