Answer:

If everything is balanced and you are still getting algae, it is likely a filtration problem. Where is the algae occurring? Start in the same place every time? If so you may have a "dead zone" where the water is not circulating making an algae bloom in that area more likely. If it is not circulating there, no chlorine is getting to the area and algae lives on again and again. There could also be algae in the filter or plumbing system that is not destroyed and it leads to recurring algae. Clean the filter if necessary.