Corals dying

[Delphinus]

Sounds totally plausible to me..

Might not be an issue in this case but something to think about as well as that a zero PO4 reading isn't always "really zero."

Early on it was claimed with biopellets that they reduce PO4 as well as NO3. However, this has not been my experience (I do use and have used pellets on tanks and have seen good NO3 reduction or elimination with them. But not PO4.) It's been claimed more recently that they can only reduce PO4 as long as they have NO3 to reduce. Unfortunately I can't claim any anecdotal evidence to support or dispute that, so that may well be the case for all I know, but ultimately, regardless if that is true or not, pellets do have a limited ability to remove PO4. So even with pellet use, having GFO on the tank is a good idea.

It's been a while since I've done my reading on this, but if you go back over the years of reef chemistry postings by Randy Holmes-Farley and others, I think you can find more info, but from what I recall: basically you have organic phosphate and inorganic phosphate. We can only test for inorganic phosphate, we can't test for organic phosphate, but it's only organic phosphate we need concern ourselves. So we test the inorganics and hope for the best that there is a proportional relationship between the two. This might not always be the case. So basically unless you have something proven in place for PO4 removal (GFO or lanthanum or whatever), you might have PO4 even if you test it out and get a zero (or nominal) reading. So even if you think PO4 isn't a problem, having GFO (or similar product) on the tank regardless, is a good idea.

(I just wish GFO was cheaper...)