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L/B Block
The reaction of La with PO4 is pretty quick but it does need some contact time as explained in the article I provided the link to. La is not something that would pool somewhere and I doubt that LaPO4 precipitate would dissolve again at normal tank pH levels. I am not a chemist so someone who is could tell you at what pH it would dissolve at. Even if it was dissolving in say a lower pH area like a sand bed or rockwork, once it got back into the water column it would recombine with free PO4, I would think. Even though you decreased the amount you are adding it doesn't sound like you have addressed trapping any residual that makes it past your fractionator. That would be the next step I would try.
Yes high PO4 can impact calcification assuming all other parameters, in particular alkalinity remain the same. Its a pretty involved topic but basically, if you increase nutrient levels and there is a resulting increase in zooxanthellae, then the demand for inorganic carbon goes up and they can outcompete the coral for it. This leads to issues with calcification. One way to counter that increased demand is to provide more inorganic carbon in the form of bicarbonate ... IOW increasing you alkalinity. There are several papers on the effects of the increase of P and N on calcification but unfortunately, they do not say anything about the alkalinity levels they were using. They basically show that the coral will grow quicker but the skeleton becomes less dense. But what if you also increased alkalinity, what effect would that have on the skeletal density? Its a questions someone needs to look at. However, these research papers are doing this work to determine what effect future ocean chemistry changes will have on corals reefs, and alkalinity is predicted to go down not up; it already has dropped over the last 30+ years.
For me, I would want to get to lower nitrate and phosphate levels by keeping the ratio around 50-100, but the oceans reefs generally have much much lower ratios. However, you need to realize that the zooxanthellae have increased in number in a high nutrient environment, so if you start limiting their nutrient sources, this will impact their number and so any change needs to be done very slowly. I have no experience in doing this, but if it was something I was looking to do, I think I would lean towards lowering both N and P levels in such a way as to maintain the current ratio where my corals are doing well.
As many have said here and elsewhere, the bottom line is how are your corals doing and under what conditions are they doing the best in your opinion. When things are going well and you decide you want to change something just because someone has come along and said a certain value or practice is better, or what I often hear is "well that is the level in the ocean", to me that is not a reason to change what is working for me. So if you find your corals are doing better at the PO4 level you have I would not mess with that.
As to which where the ratio should be on scale of 50-100, I can't tell you as I don't know. Currently our large reef runs between 98 and 125 and things look okay. I would like to lower the NO3 which is just over 40 ppm down to say 10-15 range, and I will lower PO4 at the same time, but I am currently exploring different options to lower the NO3 e.g. sulfur denitrification, ethanol filter, decreasing the number of large fish and hence feeds, water changes, adding more sand etc etc.
Hope this makes sense.
Best fishes,
C-
