I know that this is frustrating, bityou can't give up. So listen he is right phosphates are your number one problem with algea spread. So withalgea you have to remember that when you test and it is at zero, it is because your algea is consuming it. Algea would not be able to grow with out a food. So things to look at. What do plants need to grow. Lights, food source (nutrients), and water. Well since they live in water, we can go ahead and count that one out. Sothen we have lights, if your lights are old they will put a spectrum that the algea is really enjoying. 6500k is the perfect temperature for plant growth. So if you have 10k bulbs and they are old and the spectrum is slipping a little, that could be what is fueling the algea.
Next is food source. Phosphates. You need to understand how they get into your system.
There are many reasons for phosphates to appear and then be present in the water, both external and internal ones.
Some Internal Reasons
high fish and/or coral population in the aquarium,
plant material that dies off and mineralization,
breakdown of undigested food in feces,
bacterial die-off and its decomposition and mineralization,
algal die-off and its decomposition and mineralization,
any other living material that dies, decomposes,
fish slime (and coral slime in reef tanks) that decomposes (cfr Thiel)
Some External Reasons
Overfeeding and the decay of the food that is not consumed,
Phosphate in the water you are using,
Phosphate present in the salt you use to make up water,
Additives that contain it,
Carbon that leaches phosphate into the water,
Calcium hydroxide of low quality used to make limewater,
Phosphate based chelators in additives (cfr Thiel)
Phosphates in buffering compounds,
Using frozen foods and adding the liquid after it thaws, to the tank (cfr Thiel),
Using low grade calcium carbonate in calcium reactors (cfr Knop)
and many others
So to fight this problem we need to get to the bottom of it. You can use phosphate removers but that you should always look at the reason this is happening.