Okay, so my tank has been setup for a little bit over a year now. Up until about 2 months ago things were great. Healthy corals and fish no big algae problems. Then in August, I left for a month and when I came back, I started noticing brown algae growing on the rocks and glass (seen in the picture below). Wasn't too alarmed at the time, but for the last couple of weeks my LPS corals have started to look bad or dead. Fish and softies don't seem affected. I have no idea what could be wrong. I've changed nothing that I can think of. I haven't added and new livestock since like April. Any help/ideas that anyone can give would be greatly appreciated.
A bit about my setup:
Tank size- ~55 gal (29gal, 10gal tanks and 20 gal shared sump)
Age of tank- 1 year
Lighting- MH
Salinity- 1.025 (measure with hydrometer)
Water temperature- ~79 degrees
Nitrites- 0ppm
Nitrates- ~0ppm
Calcium (If applicable)- 440
Magnesium- N/A
KH-100ppm
pH- Not Sure.
Dosing anything?- No
Any recent changes to tank- No
Inhabitants (fish)- 1 osc. clown, 1 sixline wrasse, 1watchman goby.
Inhabitants (coral) - Various softies and LPS.
I have always used scripps for bi-weekly 10 gallon water changes.
I have always used RO water from the albertsons dispenser to top-off.
PLEASE HELP!!!