What are the actual observed drawbacks of long-term GFO use?

[Randy Holmes-Farley]

TLDR: GFO seems to remove PO4 much faster in a tank with coraline-covered rock than in a tank without coralline-covered rock.

I'd certainly expect that. Bare calcium carbonate surface bind a lot of phosphate that makes it much slower to drop (or raise) the water levels of phosphate. in a bare rock tank, that might include far, far more than is in the water. maybe up to 50 ppm on the rocks for only 0.2 ppm in the water.

 

[Big E]

If you don't use algae filters, do your corals keep your Nitrate in check or do you use another Nitrate-removal method?

My skimmer keeps nitrates in check. Once the corals are colonies there is more of an N03 uptake, but I believe in a form of the ammonium before it goes through the nitrogen cycle. The skimmer will take up nitrate laden bacteria and oraganic detritus, food particles,ect. before they break down.

 

[JayinToronto]

Different macroalge can have different uptake ratios, as noted above, and the ratio can depend on the water levels of N and P. I do not have data for red ogo (GRACILARIA PARVISPORA).

Here's a pdf with a bunch (same article as above). Look at table 2. It shows what you are asking about, with caulerpa much higher in N relative to P compared to other macroalgae they tested.

Wayback Machine

Thank you!