NO3 reading 50 ppm, PO4 0 ppm - Next step?

[Hooloovoo]

My tank is a few months old. It was setup in early February, but the dry rock was cured for several months before that. I noticed after cycling that my nitrates were reading really high (50 ppm with Salifert test kit). I believe this was caused by overdoing it with the Ammonium Chloride during cycling (used it to promote fishless cyling).

The past few weekends, I have performed 15% water changes, but this only seems to temporarily lower the nitrate before returning back to the 50 ppm value. My phosphate continues to read below detectable levels with the Hanna LR Phosphate Checker (just ordered the ULR Phosphorous checker).

I'm wondering what else I should do to get the nitrate down to a more reasonable level. I'm already running a refugium growing chaetomorpha algae, and I have a reactor running GAC. I have also reduced the lighting intensity and duration on the display tank to try to further increase the advantage of the algae in the refugium (I have no corals in the tank yet).

The initial diatom bloom seems to be past now, and I am no longer needing to clean the glass daily. I still have a lot of GHA, but it seems to be holding steady rather than increasing at this point. The chaetomorpha also seems to be holding steady in size after an initial rapid growth increase. Could a lack of phosphates be inhibiting the ability of the algae (GHA and Chaetomorpha) to take up further nitrate? If so, what would be the best remedy? Should I look to possibly increase phosphate? How would I do that without also further increasing nitrate?

Here are some further details about my current setup:

DT volume: 108 G
DT Dimensions: 59"L x 22"H x 22.6"W
Sump: 31 G

Refugium w/ chaetomorpha macro algae
Refugium Light: Kessil H380 Algae grow light on 18 hrs/day
Skimmer: Skimz Oval SV203 DC Internal on 24 hrs/day
Reactors:

BRS GFO & Carbon Reactor (Currently running only GAC)
CO2 Reactor (Not currently in use)​

Substrate: shallow sand bed with reef rubble
Dry Rock: 80 lbs. BRS Pukani (fully cured before adding to the tank)
Livestock: 2 Ocellaris Sp. clownfish and 1 purple tang, no other livestock yet
Feeding: Frozen Mysis - half cube per day, Julian Sprung's Sea Veggies - half sheet every other day
Lighting cycle: T5s - Off for now, Kessil A360 LEDs - Noon to 9PM @ 25% intensity, Refugium - 4 PM to 10 AM

Here are the rest of my water parameters:

Salinity: 35 ppt (Apex probe & refractometer calibrated to 35 ppt standard)
Temp: 78-79 F (Apex probe)
pH: 8.0 average (Apex probe)
Alk: ~8 dKH (Hanna Checker
Ca: 410 ppm (Hanna Checker)
Mg: Red Sea Pro Test Kit reading is above the top of the scale, accuracy is suspect

 

[Randy Holmes-Farley Verified Community Expert]

I'd suggest looking to a method such as organic carbon dosing which is focused on nitrate removal.

Nitrate in the Reef Aquarium - REEFEDITION
https://www.reef2reef.com/blog/nitrate-in-the-reef-aquarium

 

[dragon99]

You likely have some available PO4, but it's not registering on the Hanna LR.

If you really want to raise PO4, I'd dose phyotoplankton (such as Reef Nutrition) or feed some dry foods that are a bit heavy in PO4.