I'm at Proreef getting the results confirmed, and I say "Great, now people are gonna want to know how I do that" :)

No idea. Big glass box, 1/2 quality rock per gallon arranged to not trap crud, tons of SPS, lots of feeding, 50g water change every other week. Run zeo and often remember to dose the stuff and jiggle the rocks. Big ER skimmer cleaned often. Pretty much same stuff others do, nothing special.

"50g water change every other week. Run zeo and often remember to dose the stuff and jiggle the rocks."

So that's it huh? :-)

Pretty much. And "often" isn't really all that often :)

Mine both read zero right now. Sorry Brad

Cool!! How did you manage that? You've been fighting it for years :)

Which PO4 tester do you have? Hanna does two, one that measures "phosphate" (HI 713 I think?), gives a reading in ppm, and another that does "phosphorous" (HI 736) that gives a reading in ppb (parts per billion). The conversion from ppb P to ppm PO4 is "P * 3.066 / 1000 = PO4."

The 713 is not too difficult to get a zero reading, so, if that's what you have, it's very plausible considering a decent tank maintenance that you describe.

With 736 it's very sensitive and the margin of error at that small a resolution is impossible to get away from, so a zero reading with the 736 is more impressive. I can actually track PO4 fluctuations in my tank using the 736 on an hourly basis ("wow, some fish just took a really large poop.")

With any of the Hanna testers, the likelihood of a false positive is larger than a false negative reading. Ie., a fingerprint, air bubble or particulate in the vial will register as a colour change and give you a higher reading than real. With the 736 you have to really avoid shaking as the microbubbles can take longer than 3 minutes to dissipate.

I finally added GFO,it's made a huge difference. I still see the PO4 struggling to come back but I'm changing the GFO out weekly (Brett does the same with his). Just a cup and a bit per week (well below the recommended amounts, but well above the recommended changeout period). The nice thing is that by using the Hanna tester I can tell when the GFO is spent, and that way I know I'm not wasting GFO (stuff's expensive) by changing it out weekly.

I still have the zeo going for now, but I like that I don't need to do it as much. For whatever reason the zeo was not touching PO4 in my tank. I see sort of two possibilities for this:

1) Zeo doesn't do as much for PO4 as we might think, maybe there is a problem of scale (ie., my system is closer to 400g total, but I had great results with zeo alone when the system was more like 120g total), or,

2) NO3 was reading zero for me for a long time, maybe PO4 reduction can only occur at the same time as NO3 reduction (I think it's been theorized that NP pellets work in this manner) so basically NO3 is the limiting variable, once it cannot be reduced any further, no PO4 reduction can occur.

.. just my thoughts anyhow.

PS. I also switched to 50g weekly changes, from 24g weekly changes, and this has also yielded positive results. It was apparent within 2 waterchanges that this was a good move.

Good to hear you're finally winning that battle!

Did you put the fish back in yet? I'm thinking leaving your tank fish less for had something to do with the zero reading?

Half the fish were in, the rest just went in now. But I fed the tank even without fish, as I have a lot of hermits, shrimps, etc...

I've always had low readings though, but I did expect some PO4. I'll test again in a month and see what the full fish load does.