Ok here we go... imo... if you try the following... you will see remarkable improvement in one week & maybe a near cure in two...
1st... pull & rinse the bio-media every month (others will say ditch it, but mine looks like new & is not used as a biological filter, but only as a spacer to allow for flow & for the carbon to sit on top of, a real key in the rsm aio - my Live rock & sand is my bio filter) run hot water over it in the sink, until clean.
2nd... replace the carbon... buy & use in it's place the 11 oz Chemi-Pure Elite carbon (the Elite has GFO to reduce nitrates) - the manufacturer says the chemi pure will last six months - replace it 4 times a year - every three months... the is a big key...
3rd... for one week turn your lights on for only 5 hours a day
4th... perform a 30% water change now, siphon & suck up what you can... and scrape & then do 10-15% water changes every week ( 5gals)
5th... start feeding every other day & only what they can consume in two minutes before the food can fall to the bottom - a very very small amount...
6th... after one week, run your light no more than 7 hours a day for the next month
7th... add a better cuc - I suggest 20 snails... Ceriths, Nassarius, Margarita Turbos, Astrea & Banded Trochus
8th... rinse your sponges at least twice a week...
give this a go... and I think... you will be very happy with the results
it works for me... this may sound like a lot... but I don't spend 15 mins a week in tank maintenance... and have zero algae... or just enough to feed the snails