Algae problem

i was gone for a week letting my brother take care of my three reefs, 90 55 and 30. i came home 2 days ago and now my 30 gallon has a huge algae break out like there are so many different kinds of algae in my tank...but my other two reef aquariums are perfectly fine...i checked my water parameters and everything was fine.
ph: 8.2
salinity: 1.23
ammonia: 0
nitrites: 0
nitrates: 20
Kh: 8-9
mag: 1250
cal: 400
phos: 0
i dont understand what he could of done to have this huge out break of algae. from what i can see there are diatoms,green slime, red slime, hair algae, Bryopsis pennata, bubble algae, red bubble algae, Centroceras clavulatum algae, and some weird neon green stalks. is there anyway that i can get rid of these algae safely without harming my corals(i can relocate them to my other reefs), i have mushrooms, colony polyps, kenya tree, goniopora, and toadstool. also i have about 4-5 watts per gallon using T5 lighting for 9-11 hours, tank is roughly 7 months old. so is there anyway i can get rid of these algae safely and what can i do to prevent this from happening in the future because i have never had a algae breakout in any of many reef tanks until now.

 

Probably your bro fed your tanks every time he wanted to eat himself. Check your fish food supplies - it will give you an Idea check phosphates and NO3, lights off for a couple of days and water change.

 

Sounds as if someting was spilled into the tank, food/suppliment wise. Another possibility is something died. Did you check the ammonia?