Could Emerald Crabs be partially responsible for this? other cause?

[Outlets710]

After doing a 50% water change my PO4 came down to only .78 from .92, I thought it would at least bring it down to the .5 or better. How aggravating. I guess this didn't happen over night nor wil it be corrected over night.

I am from the UK, so we mainly use Rowaphos which is a premium brand and is available worldwide. In the USA, there are mainly brands, but the stuff from Bulk Reef Supply should be fine for you:http://www.bulkreefsupply.com/bulk-media-additives/gfo.html

Reducing the PO4 to around 0.03ppm is good enough. Reducing NO3 to less than 5ppm is a good target.

The main ways to reduce NO3 and PO4 is lots of regular water changes and use of GFO. Maybe use some GAC (Carbon to soak up some organics etc).
Carry out 10% weekly water changes - at least for the next 3 months.

Try your best to siphone out the diatoms/Cyanno. - Use an airline hose stuck to a tong to reduce the amount of water loss

Perhaps, also reduce your light hours to no more than 6 hours. Your corals will be fine with this. SPS corals may brown slightly, but certainly wont die.

All the while, watch your Calcium, Alkalinity and Mag levels.

I just noticed that your specific gravity is stated to be 1.023.

Try and compare with a friend or LFS with a healthy tank and or calibrate your refractormeter and try this to about 1.025.

The one other thing you can do is do lights out for 3 days every 5 days to really hit the diatoms and Cyanno if it starts getting worse.

My first tank was a nano. It is testing. But persevere and in 3 months your tank should be fine.

I will order tomorrow from BRS,also slowly raising salinity as many have suggested.
I was curious about siphoning,if it's beneath the sand bed it won't do any harm or release any toxins if surfaced? right now I'm lucky , very very little is on the rock if any. I would love to clean my sand bed however I don't want to expose any more of it to the water or does it not spread that way?

Thank you for the in-depth reply