Well I finally broke down and added GFO to the system this week, and the results so far are .. well .. astonishing. Or at least, suffice it to say, I'm not sure I understand what's going on.
On Sunday I added 16 Tblsp (1 cup) of GFO to a TLF 150 reactor. Being concerned of a sudden PO4 drop (as this is reasonably well documented to be stressful to corals and can cause tissue recession and so on), I started with what I thought was a conservative amount of GFO (high capacity GFO, sourced by Eli at CaCO3Reef). Established guidelines for HC-GFO seem to suggest starting around 8 Tblsp per 8 gallons (before eventually being able to double that) so for a system with 300 gallons you'd think you'd want 37.5 Tblsp, 1 cup is 16 Tblsp so this is less than half of that so here I was thinking I was starting off with "half of the half amount for starters."
Set the flowrate to the reactor to slow (estimate maybe 50ml/min). At 50 ml/min it is basically a slow trickle. If I have my math right, it would take about 16 days to fully turnover one tank volume at this flowrate.
I thought it would be fun to test the tank PO4 and the reactor's output of PO4 for the first little bit. Doing dual tests like this should tell me a couple things:
1) If the GFO is "working" at all
2) How fast the PO4 is dropping in the tank
3) When the GFO is expended and needs to be replaced.
.. All good things to know.
Soooo anyhow, here are the results so far:
Day 0: Tank PO4=0.17ppm Reactor PO4=0.15
(I thought, oh good, it's only lowering the PO4 a little bit, this should be a nice safe slow reduction.)
Day 2: Tank PO4=0.08ppm Reactor PO4=0.18
(I thought, huh? Ok, I did some bad tests here. Try again tomorrow.)
Day 3: Tank PO4=0.07ppm Reactor PO4=0.00
(Makes me think the reactor testing from the day before was the bad result.)
So um ... Wow? Does this even make sense? That GFO must be one heck of a PO4 scavenger if it has halved the PO4 in the tank already after 2-3 days at such a small volume and small turnover.
So far there is one coral exhibiting some slight tip burn. It is very minor at this point however. It should easily recover assuming it doesn't get worse first. The piece in question is a Cali tort which hasn't actually been blue in over a year now (it did phenomenally well in the first quarter of the first year and then has been struggling ever since. It is clearly a strong coral however though, because in this time any other coral that has shown the slightest bit of distress over the PO4 eventually RTN'd at some point.)
I'm really excited to see where the reduced PO4 levels should take me in terms of coral growth and colour. I hope this is a positive step for the tank. It would be nice, for example, to see the Cali tort start to look like an actual Cali tort again.
Oh, the possibilities!