i as well would like to know what you have changed lately. Something must have changed because your tank has been up for 5 years, you would have noticed algae long ago. You did mention you need new filters for the RO unit, are you using Tap water now and used to be using RO?? if this is the case, forget about what your spending and just get the filters lol. I know cost always sucks as a factor but sometimes you just need to spend the cash.
What kind of protein skimmer are u running.
At this point testing your water is worthless, we know you have a nutrient issue.
Feeding should be cut back to once every 3 days, of course depending on your fish list. Some fish require daily feedings, lets hope you dont have any of these.
Lights should never exceed 8 hours for Daylight bulbs, as this is only asking for trouble. What type of lights do you have and are you housing corals?? if so what kind??
I realize all of this has already been said but i would get on this right away. Manual removal during your water changes, and yes larger water changes with RO water, just for a few weeks should help keep it down until all the nutrients are out of the water column, and with the addition of a more strict feeding and lighting schedule, things should start to shape up soon for you.
have you considered maybe plumbing in a HOB refugium. There not normaly the greatest thing in the world, the HOB units anways, but having some of those natural plants in there is great for all tanks, new or old. If room permits, why not try and get something along those lines. Cost will be an initiall factor, but it pays itself off in salt lol. Exspecially when you doing around 10 gallon water changes with saltwater for two weeks.