Filtration / ULNS

Hi I really need help with my sump.

My tank is an upgrade filled with stock that's approx five years old. Everything is progressing fine.

But, my sump at the moment is just a tank to hold stuff. Its a basic sump, three compartments. First skimmer and a heater, second empty at the moment apart from algae growing on the glass, third a heater, phos reactor and return pump.

I did try cheato in the centre section but although it survived it did not grow! I got a fair bit of hair algae on the...