Algae problem

Your using direct sunlight? Theres your big problem.

 

I'm using that in my big tank for many years. I use mirrors to get the sun in my tank. I never had any problem with that.

 

Cant say for sure if direct sun is your problem but I am pretty sure every aquarist and every book has taught me to place a tank in an area away from direct sunlight otherwise algae would be an issue.
It may have been fine for years but if something in your tank randomly gets out of whack then it will certainly be a problem. Example. Your phosphate and nitrates have been 0 for years and you had no source of nutrients for an algae or any sproes of gha in your tank-direct sunlight no issue. Now take that tank introduce a few sproes from your frag and something caused a nitrate or phosphate spike, now you have a hgue issue because your lighting supported it all along but your other factors held it in check, now that those other factors are also favorable for algae the perfect light you always had makes a HUGE and fast algae bloom