personally I would do a complete overhaul.
Steps I would take:
- Remove all the rock, put it in a bin with saltwater, a heater and a powerhead.. Cover it so no light gets through and forget about it for a few weeks, changing the water ever so often if lots of debris comes out of it. It's probably the best way to get every spec of algae off the rock
- Get rid of the deep sand bed and instead leave about a 1" bed at the bottom. sandbeds can become finicky through time. A 4 year old deep sand bed might be doing more harm than good
- 50% water change when removing most of the sand
- If you'll dedicate yourself to change the filter pads weekly, then I'd put some in the cannister filter.. if you don't foresee yourself doing so, then I'd put nothing in the cannister (besides some carbon from time to time)
- Cut the lights down to like 4 hours a day. Corals will live much longer in low light than they will with too much light.
- if an RODI is within your budget, I would definitely invest in one, but in the meantime you should check your water to make sure that isn't the issue.
But that's just me. . good luck!