Hi there Willy43,
The exact same thing that has happened to you has happened to me and I'm sure plenty of reefers out there. We create an environment that is too clean and too unnatural to sustain life.
I have a 100 gallon SPS dominated tank and had "two" gfo reactors running with undetectable NO3. The reason was because my rocks and deep sand bed (at the time) had phosphate poring out of it at a lethal rate.
The first two reactors were the GFO, the third reactor is Purigen and the fourth is carbon.
Well the reactors were so efficient that all the water that went into the units was stripped from phosphate. I had the reactors running for six months and they were able to maintain or keep up with the phosphate that was oozing out of the rocks and sand.
So I later reduced the sand bed to 1.5 inches in an effort to control the algae and cyanobacteria and kept the reactors going for another 3 months as the rocks were still releasing PO4 all until the STN started happening throughout the most sensitive SPS in the tank. Well long story short the rocks no longer release phosphate and I have not used the reactors for the last eight months. I increased the Bioload with plenty of fish and daily feedings (and I really FEED...as I like seeing the bellies... sorry Latin thing "big bellies = healthy"). In the event that the phosphates go up (which hasn't happened for the last four months) I add phosphate remover. My nitrates have gone up and I've been dosing the tank periodically Red Seas NO3
O4-X with great success to maintain it at a healthy low level along with water changes every three weeks.
Here is the tank a week ago
My suggestion would be less is more. Don't do any water changes yet and let the tank get "dirty", corals are more resilient than we think.
Hope that helps!
Jorge
