lights, filtration, temperature, thats what i am seeing as a problem right now, also you didnt add in your parameters you said they are perfect but at what levels, what is your ammonia, nitrites, nitrates, phophates, calcium, ph, and kh? these are all things that we need to know even a few slightly elevated readings can compound when you take them all into consideration at once . if the lights you are referring to are normal freshwater flourecents then that is a mian problem. if you can't remember your filtration then it probably isnt adequate. You probably need a protein skimmer, as well as what you have now, or an upgraded version of what you have now. Your tank should be at about 79 or 80, that will also help kill off some of the algae. Instead of the 20% monthly do 10 percent weekly. it will go a long ways to helping reduce your nutrient levels. i am speaking out of experience, when i started my first salt tank i had a 29 with a heater, saltwater and a hob filter. Now i am breeding saltwater fish, and propogating corals on a daily basis. time and patience are two more key factors. one more thing what kind of salt are you using? hope i can help.... let me know and i will keep trying to help