Hi Fishfreak, adding more product
might help to reduce phosphate. I say "might" because the bacteria would have to colonize your system and grow well before you'll observe any substantial reduction of PO4. I don't want to present you with unnecessarily grim odds, because we hear of great results for this particular application all the time. But it does require some things, namely a large surface area of suitable anaerobic microhabitat (Siporax, old AquaChar, deep sand/gravel bed, etc.). If they receive some light, even better, as this will roughly double their productivity. Carbon dosing can help, especially if you don't have a particularly high concentration of DOM (they prefer vinegar to alcohol or sugars).
The last factor is not as much within your immediate control; your existing microbiome is unique, and therefore it's difficult to predict how much competitive pressure a seed culture will encounter from other bacterial species. Here's where more product really can help; when seeding, it's probably best to add the dose directly to the biomedium (as to really concentrate it). Large, concentrated doses help to inundate a habitable area in terms of sheer numbers of cells. If more cells successfully recruit to some given area, their collective release of antibiotics and other antimicrobial substances is more effective in creating a so-called zone of inhibition against competing species. Here's where dosing for seeding is very different than dosing for feeding (at least the way I like to recommend them): For seeding, dosing should be in pulses as to take advantage of strength in number, whereas for feeding, dosing should be in smaller but more frequent additions.
But hey, export is only half of the equation. Ultimately, there is some limit to how low your PO4 can go, and import will be exactly as much of a factor there as export (as I'm sure you already know). So be prepared to set expectations accordingly. Sounds like you feed you tank quite generously, so the limit may be pretty strict. Even if these bacteria exhibit a limited capacity to remove PO4 in a particular system (due to competition, or UV sterilization, or whatever), they are nevertheless an incredibly nutritious food/probiotic that reduces instead of increases phosphate levels.
These bacteria are commonly used in industrial and agricultural wastewater applications and demonstrate an ability to remove far more PO4 than you're seeing; however, they do need the living space and must be able to defend it well enough to proliferate.
Do you keep a refugium for that caulerpa? Sound like a great place for some dedicated biomedia, because of the light and the extra DOM.
Hope some of this was helpful!
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