SPS issue

Yes a rapid drop in PO4 levels can stress corals. Usually it has to be more than 1ppm drop to act as a significant stressor. Typically when first starting GFO I suggest using a very small amount and then replacing that in about a week or two. on a system your size I would use a 1/4cup and then replace it with a 1/2 cup in two weeks or so.

 

Keep a close eye on your alk also. GFO will lower it and if you are running to much it will lower it drastically.

 

yeah, give it a rest for a week and then start with a small amount in another week.

 

Find the source of your nitrates. If it's not from over feeding try blowing off your rock work and look for piles of poo in the back corners of your tank and sump.

Time to step up the water changes for a bit until you get them back down to a more desirable level.

 

small weekly water changes are good for trace element supplement and reducing nitrates. But they're not as effective as larger water changes at removing nitrogenous waste. Allow me to illustrate this point: suppose you have 100ppm nitrates, you do a 10% water change then NO3=90ppm, you do another 10% water change NO3=81ppm, another 10% NO3=72.9ppm, another 10% NO3=65.6ppm, yet another NO3=59.2ppm. All the while new nitrates are being created between water changes. It's small but cumulative. If you would have done 50% to begin with your nitrates would be 50ppm instead of 59.2ppm. DO NOT DO A 50% WATER CHANGE (it's stressful and unnecessary, I'm simply using this as an example). DO do (haha dodo) a quarterly 30% "flush" on top of small weekly water changes. Perhaps drop your weekly to just 10 gallons instead. Think about adding a refugium if you don't have one already.