Ok, here's my humble opinion. Your problem probably is being added from a couple of places but the most possible cause is via water supply (unless you are feeding like a mad man). I say this because when i first started. like most i was using treated tap water. initally everything would look great, then over time it went all down hill with algae and such. Now i strickly use RO/DI. notice it's not just RO. The added DI portion of the filter does turn color indicating that it's filtering the ions that are present in the water. A simple test is to buy a TDS meter and test your water. it's relatively inexpensive and i can almost gurantee you you're adding unknown pollutants into your tank. here's a link to a tds meter that cost only $20 Aquarium Water | Reverse Osmosis | RODI Water | ReefKeeper Water | TDS Meter from Senno
. My tap water measures about 250 to 350 TDS before RO filter. After RO it measures like 6 TDS, then after the DI it measures 0TDS (TDS -Total Dissolved Solids (TDS) are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm). TDS is directly related to the purity of water and the quality of water purification systems and affects everything that consumes, lives in, or uses water, whether organic or inorganic, whether for better or for worse.) Strive for ZERO 0!
Alot of hobbiest don't like to spend the $ on a RO system but i will assure you it's well worth the investment! If you consider the starting over cost, sand, fustration, cleaning, etc and i can go on and on, it's well worth it. If you cannot afford a new RO, go to your local goodwill store, habitat for humanity store frequently and you will walk up onto a RO that someone sold/gave to the store for cheap! we've witness this at several visits here locally. my friend got a full ro system for under $15. then he changed the filters and everything came to about $65 and works as good as new. if you do alittle research you will be able to figure out exactly what is needed and how to aquire it cheap. heck if you like, i'll tell you what is needed.
i'm surprised you stated that the urchin isn't eating algae. what type of algae is it? i know first hand that they do eat algae because i saw it with my own eyes!
other than that here's what you can do to help fight algae.
1. change bulbs if they are old
2. make sure you use quality waterfrom here out for topoff and SW making (RO/DI or at least distilled water from a grocery store. This will also help prevent this from happening again)
3. lower you photo period until you control the algae.
4. Use Rowa Phos or some type of GFO or Granular Ferric Oxide base type phosphate removal material (don't use aluminum base type like phosguard as this leaches aluminum into the water)
5. Patients, lots of it because you won't see improvements for quite awhile.
6. Hope you are using a quality skimmer.
7. feed fish and corals very little (most people overfeed and this is where alot of problems can come from)
8. CUC you can also put in a sea urchin and he will go to town bigtime! if you do please return him back to a LFS otherwise when all algae is gone he will starve to death (this is one way to see rather quick results but you must do all of the above to prevent this algae issue again)