Need tips for finding the source of Nutrients

[Slick Fork]

Hi there,

So, I've got Algae and therefore nutrients... there, I've admitted it. What's killing me though is where are they coming from and what am I doing wrong to remove them.

So, a the background info:
Tank 33 gallon
Sump ~30 gallons actual volume
Live rock ~I THINK it's about 30 or 40 lbs. 90% of it was dry 2 year ago when I started the tank up.
Skimmer: Vertex In-180
GFO and carbon mixed in a reactor
Chaeto in a stagnant part of the sump with a PC (13 watt) Flood light on it.

Flow in the display:
2x6055 opposing and set to pulse, creating a small 3/4" wave
1 6045 with the wide flow mod trying to fix stagnant areas at the bottom of the tank
Return pump ~500gph

Inhabitants
Tail spot Blenny
Sailfin Goby
Yellow Watchman Goby
Neon Blue Goby
Royal Gramma
Peppermint Shrimp
SPS/LPS/Acans and a struggling zoanthid frag

Maintenance:
5 gallon water change every two months with H2Ocean salt - RO/DI water at 0ppm
Calcium added to the salt during water changes
on my 3rd month of Prodibio (only using 1 vial, not the recommended two.. because I'm cheap)
Coral Snow randomly (when I remember)
GFO Changed - not often enough, every couple of months.
When I have time, I vacuum detritus up from the display through a filter sock in the sump (Sock is only there temporarily)

Feeding:
NLS pellets and Spirulina pellets a little less often than daily. No Frozen food.

Relevant history and other information:
-Was on that Royal Nature salt last year, switched to D&D H2Ocean in January(ish) and the tank has been improving ever since.
-Couple of skimmer overflows dumping a weeks worth of skimmate into the sump each time in the last month (likely culprit?)
-Chaeto grows excruciatingly slowly... maybe due to low flow, and older/smaller lightbulb?

So, my hunch is that the two big skimmer overflows are the prime culprit in my most recent algae bloom, but I have a sneaking suspicion that since the chaeto isn't tumbling, that whatever benefit I get from the top getting the light is offset or worse by stuff decomposing on the bottom of the ball.

What else am I missing? I kind of feel with a skimmer rated for my old 180 gallon system and frequent water changes, I shouldn't have any nutrient issues at all. It doesn't seem like I have a huge bioload, and I'm certainly not feeding a ton.

Would really appreciate some suggestions

Cheers